General and specific combining ability of male blue catfish (Ictalurus furcatus) and female channel catfish (Ictalurus punctatus) for growth and carcass yield of their F1 hybrid progeny

Aquaculture 420–421 (2014) 147–153

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General and specific combining ability of male blue catfish (Ictalurus furcatus) and female channel catfish (Ictalurus punctatus) for growth and carcass yield of their F1 hybrid progeny Brian Bosworth ⁎, Geoff Waldbieser Catfish Genetics Research Unit, USDA-ARS, P.O. Box 38, Stoneville, MS 38776, USA

a r t i c l e

i n f o

Article history: Received 5 December 2012 Received in revised form 17 October 2013 Accepted 22 October 2013 Available online 31 October 2013 Keywords: Hybrid Catfish Combining ability Performance

a b s t r a c t U.S. aquaculture production of channel catfish female (Ictalurus punctatus) × blue catfish male (Ictalurus furcatus) F1 hybrids has increased substantially due to the hybrid's improved growth, survival, carcass yield and ease of harvest compared to the more commonly farmed channel catfish. However, information on the genetic architecture underlying phenotypic variation in the hybrid which is needed to develop an efficient genetic improvement program is lacking. Progeny from two separate factorial matings between male blue catfish and female channel catfish (ten males × seven females and twelve males × five females, respectively) were grown communally in earthen ponds and measured for carcass yield and weight at approximately 520 days posthatch. Parentage of progeny was assigned by inheritance of microsatellite genotypes. Progeny from 118 full-sib families, within 12 maternal half-sib and 22 paternal half-sib families, were measured for harvest weight (n = 1288) and carcass yield (n = 1101). Variance component estimates associated with dams (dam general combining ability and heritabilities based on dam half-sib families) were high, variance associated with sires (sire general combining ability and heritabilities based on sire half-sib families) were intermediate, and variance components associated with the dam × sire interactions (specific combining ability) were low for carcass yield and harvest weight. Data indicate that the genetic variance for carcass yield and harvest weight of F1 hybrids was primarily additive. Selection for improved carcass yield and growth based on additive genetic merit of blue catfish male and channel catfish female parents should be effective for improving performance of hybrid progeny. Published by Elsevier B.V.

1. Introduction Commercial catfish farming is the largest segment of U.S. aquaculture, but catfish production has decreased approximately 50% since 2003 primarily due to increased feed costs and competition from imported Pangasius and channel catfish (Ictalurus punctatus) from Southeast Asia. U.S. producers must reduce production costs in order to remain competitive in a global seafood market. F1 hybrids produced by mating female channel catfish with male blue catfish (Ictalurus furcatus) have better growth, survival, meat yield and ease of harvest than the traditionally produced purebred channel catfish (Dunham et al., 1990; Green and Rawles, 2010; Li et al., 2004). Improved spawning and hatching techniques have allowed more efficient production of hybrid catfish fry and current production records indicate that hybrids will comprise 30% to 40% of the U.S. catfish harvest in 2013. Although hybrids generally perform better than purebred channel catfish, there is potential for further improvement of economically important traits of hybrids through genetic selection in the parent species. However, little is known about the genetic architecture underlying ⁎ Corresponding author. Tel.: +1 662 822 8022. E-mail address: [email protected] (B. Bosworth). 0044-8486/$ – see front matter. Published by Elsevier B.V. http://dx.doi.org/10.1016/j.aquaculture.2013.10.026

phenotypic variation for traits of channel catfish × blue catfish hybrids. Understanding the genetic basis for phenotypic variation of important traits will allow design of efficient breeding programs to further improve hybrid catfish performance. Genotypic value is a function of the additive effects of the two alleles at a locus independently and a dominance deviation due to possible interaction among the two alleles (Lynch and Walsh, 1998). Quantifying the relative contribution of additive and dominance genetic variance to the phenotypic variance for important traits is critical to developing efficient crossbreeding programs. Sprague and Tatum (1942), in work related to corn breeding, defined general combining ability (GCA) as the average performance of a line in hybrid combinations and specific combining ability (SCA) as the deviation of certain crosses from their expectation based on the average performance of the lines involved. GCA is primarily a measure of additive genetic effects common to a line and SCA is a measure of dominance genetic effects in their crosses. The theory developed for GCA and SCA of inbred lines can also be extended to individual plants and animals. The concepts of GCA and SCA have been widely used to guide development of breeding plans for many plant and tree species (Li et al., 1995; Schrag et al., 2009; Volker et al., 2008; Wu and Matheson, 2004), and to a lesser extent animals and fish (Hörstgen-Schwark et al., 1986; Ooi et al., 1975; Wang et al.,

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2006; Wheat et al., 1981). Wang et al. (2006) estimated GCA and SCA for parents of hybrid striped bass (Morone chrysops female × Morone saxatilis male), a breeding program similar to the hybrid catfish model. Heritability, defined as the proportion of total phenotypic variance due to additive genetic variance, is widely used to guide breeding decisions in plant and animal breeding (Falconer and MacKay, 1996). Similar approaches could be used to estimate the relative magnitude of additive (GCA, heritabilities) and dominance (SCA) genetic effects for traits of hybrid catfish. Catfish, like many fish species, are highly fecund and have external fertilization allowing easy manipulation of large numbers of both male and female gametes. Easy access to eggs allows implementation of mating designs originally used in plants (factorials and diallel crosses) for estimating additive and dominance genetic effects. Factorial mating designs (all males mated to all females), also referred to as North Carolina Design II (Comstock and Robinson, 1952), provide estimates of the relative importance of additive genetic effects for both sires and dams (GCA and heritabilities)), dominance effects (SCA), and maternal effects. Initial analysis of hybrid catfish produced from factorial matings will be informative in determining the genetic basis for trait variation and guiding development of an efficient genetic improvement program. The objective of this study was to estimate relative importance of additive and dominance genetic variance for growth and carcass yield of hybrid catfish progeny produced from factorial matings of blue catfish males and channel catfish females. 2. Materials and methods 2.1. Matings A complete factorial mating design (every male mated to every female and every female mated to every male) as described by Comstock and Robinson (1952) was used to produce maternal halfsib, paternal half-sib, and full-sib families of channel catfish female × blue catfish male F1 hybrids. Matings were conducted as two separate factorials: factorial 1 had 12 male and five female parents and factorial 2 had 10 male and seven female parents. Two separate factorials were used because 1) we needed sufficient egg numbers dedicated to each full-sib family for adequate fry production given potential differences in fertility and hatch, and 2) blue catfish are less polymorphic than channel catfish at the microsatellite loci used for parentage assignment and limiting the number of blue male parents within a factorial improved the ability to assign parentage. Based on experience producing hybrids we felt that eggs from a single female could be split into about 12 equal aliquots and give sufficient fry per full-sib family. Therefore, 10 and 12 sires were used in the two factorials, respectively. Number of dams was limited primarily by hatching and rearing facilities, and costs associated with increasing the total number of progeny genotyped. In April, 2010, 22 mature, D&B strain male blue catfish were transported from the Catfish Genetics Research Unit, USDA-ARS, Stoneville, MS to Ben Hur Farm, Louisiana State University, Baton Rouge, LA and held overnight in raceways with flow-through groundwater and aeration. The following day males were killed by cranial percussion; testes were surgically excised and macerated in Hanks Buffered Salt Solution to produce sperm suspensions. Sperm suspensions were cryopreserved in straws (Cryobiosystems Co., Paris, France) according to Hu et al. (2011). Straws were labeled with the pit-tag number of the donor blue sire and stored in liquid nitrogen until use. In May, 2010, mature female channel catfish originally obtained from three commercial farm populations and grown to maturity at the Catfish Genetics Research Unit (weight range 2.1 to 4.0 kg) were selected for hormone induced ovulation based on external phenotypic indicators of reproductive readiness (distended abdomen, reddish vent) (Dunham and Masser, 2012). Females were held in concrete raceways with flow-through well water at 26 to 27 °C and 5 to 7 ppm dissolved

oxygen. Females were injected IP (intraperitoneal) with a priming dose of 2 mg/kg carp pituitary extract (CPE) suspended in sterile saline followed by a resolving dose of 8 mg/kg CPE 20 h later (Dunham and Masser, 2012). Females were checked for ovulation about 26 h after the second injection and females that had ovulated (eggs flow freely with abdominal pressure) were removed from the raceway, anesthetized in 100 ppm MS-222 (Western Chemical Inc., Ferndale, WA), rinsed in hatchery water, dried with a towel, and eggs were expressed by abdominal pressure into a 4-l plastic bucket coated with non-stick cooking spray to prevent eggs from adhering to the bucket. Total egg volume for each female was measured with a 1-l volumetric cylinder and eggs from each female were split into approximately equal aliquots (typically 40 to 60 ml of eggs per aliquot), based on the number of males designated for the factorial. Each egg aliquot was placed in a small plastic bowl (~250 ml capacity) coated with non-stick cooking spray. A single straw of sperm was used to fertilize each egg aliquot. Just prior to use for fertilization, sperm straws were placed in a water bath at 40 °C for 20 s to thaw. After thawing, ends were snipped off the straws, sperm solutions were added to egg aliquots, and a volume of water about three times the egg volume was added to activate the sperm. Individual egg aliquots were fertilized with single-male sperm rather than using pools of sperm from several males to reduce potential differences in fertility related to differences in concentration or motility among sperm samples (Wang et al., 2006). Approximately 5 min after fertilization, all egg aliquots from a female were combined in a plastic bucket and 1-l of hatchery water containing eight gel dissolving units of the proteolytic enzyme bromelain was added to the eggs and stirred for approximately 3 min to eliminate egg adhesiveness. Eggs from each female were hatched in a separate McDonald hatching jar supplied with groundwater [6 l/min, 27 °C, 7.0 ppm dissolved oxygen (D.O.)]. Matings for the first factorial were conducted on May 12, 2010 and matings for the second factorial were conducted on May 19, 2010. Every male was mated to every female within each factorial. 2.2. Fish husbandry Immediately after hatch, the volume of fry from each female was measured to the nearest 2 ml with a 100 ml volumetric cylinder. Approximately equal volumes of fry from each female within a factorial were pooled in two replicate 160-l fiberglass tanks supplied with flow-through groundwater and diffuse aeration (3 l/min flow and 5 to 6 ppm D.O.). Fry from all females within a factorial hatched within a 36 hour time period. Fry were fed 55% protein finfish diet (Zeigler Brothers Inc., Gardners, PA, USA) at swim-up and for approximately 10 days following swim-up. Fry from each replicate tank were then stocked in separate, fertilized 0.04 ha ponds (two replicate ponds per factorial). After transfer to ponds, fry were fed salmon starter at a rate of 49 kg/ha per day split into three equal feedings until fish were observed feeding at the surface, typically about three weeks after stocking. When fish began to feed at the surface they were fed a 35% protein floating commercial catfish fingerling diet (Delta Western, Indianola, MS) to apparent satiation twice a day Monday through Friday and once a day on Saturday and Sunday through October. During the winter (November through March), fish were fed once daily if afternoon water temperatures were warm enough to stimulate feeding activity (~ above 15 °C). In April 2011, fish from each pond were seined and restocked at 14,820 head per ha in 0.1 ha ponds. After restocking, fish were fed a 32% protein commercial floating diet (Delta Western, Indianola, MS) to apparent satiation once daily. Fish were fed through October of 2011. In early November, 2011 ponds were harvested by seining and approximately 900 fish from one pond from each factorial were randomly selected for trait measurement and parentage assignment. Fish from the first factorial were harvested and processed one week before fish from the second factorial.

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2.3. Data collection and analysis

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factorial mating design used in this study, general combining abilities for sires and dams are equivalent to the sire and dam variances, respectively; and specific combining ability is equivalent to the sire × dam variance (Cotterill et al., 1986). Heritabilities for carcass yield and harvest weight were estimated based on variance associated with half-sib sire families (h2s ) and halfsib dam families (h2d) using the following equations:

Hybrid progeny selected for data collection were transported to two concrete raceways supplied with flow-through groundwater (40 l/min, 27 °C) and diffuse aeration (D.O. ~5 ppm) and held for approximately 18 h prior to processing. At processing, fish were removed from the raceways, stunned by electricity, and implanted with a PIT tag in the caudal musculature. After tagging, a piece of barbel ~1 cm long was collected from each fish, placed in a 1.5 ml polyethylene tube labeled with the donor fish's tag number, and frozen for subsequent DNA isolation. Fish were then weighed to the nearest 0.5 g, gender was recorded, and fish larger than 400 g were processed to determine carcass weight. Processed fish were beheaded with a Badder 166 (Baader North America, Kansas City, KS, USA) and viscera removed by hand. Carcasses were weighed to the nearest 0.5 g and carcass yield (100 × carcass weight / whole fish weight) was calculated. All fish from factorial 1 were processed the day after harvest, approximately half the fish from factorial 2 were processed the day after harvest and the remainder were processed two days after harvest. Genomic DNA was isolated from blood of parents and barbels of progeny, and genotypes of potential sires and dams were produced from 16 microsatellite loci (Table 1). Primer sets and amplification conditions used have been reported previously (Waldbieser and Bosworth, 2013). Parentage was determined from inheritance patterns of alleles at these microsatellites using Cervus v3.0.3 software (Kalinowski et al., 2007). The following statistical model used was for analysis of growth and carcass yield:

where σs2 = variance due to blue sire, σd2 = variance due to channel dam, σs2× d = variance of sire × dam interaction, and σ2e = error variance. Standard errors for heritability estimates were estimated by Taylor series expansion (Hohls, 1996). BLUPs for GCA of harvest weight and carcass yield for individual sires and dams were estimated using the Mixed Procedure of SAS and correlations among these GCAs were used as estimates of genetic correlations for harvest weight and carcass yield in blue sires and channel dams. GCAs and heritabilities based on dam variances reported in the results may be inflated because additive genetic and maternal effects are confounded in these estimates. In addition, genetic variance was likely higher in the dams than sires given the more diverse nature of the dam used (three populations) relative to the sires (one population). The effect of female population was included in an initial analysis and found to be insignificant for carcass yield or harvest weight and was not included in the final analysis.

Yijklm ¼ μ þ Si þ D j þ ðS  DÞij þ Gk þ Fl þ eijklm

3. Results

where Yijklm is the trait value for the mth hybrid progeny, μ is the overall mean, Si is the random effect of the ith sire, Dj is the random effect of the jth dam, (S × D)ij is the interaction between the ith sire and jth dam, Gk is the fixed effect of the hybrid progeny's gender, Fl is the fixed effect of the lth factorial and eijklm is the random error. Analysis for carcass yield included additional fixed effects of total weight as a covariate and processing day within factorial since fish from the second factorial were processed on two consecutive days. Differences among fixed effects for growth and carcass yield were compared with the Mixed Procedure of SAS (version 9.1, SAS, Cary, NC). REML estimates of variance components for random effects (sire, dam and sire × dam interactions) and the associated variance–covariance matrix used to estimate their standard errors were estimated using the Mixed Procedure of SAS. For the

A total of 1697 hybrid catfish progeny were genotyped at 16 microsatellite loci in three multiplexed panels. Names of microsatellite loci for each panel, number of alleles in parents and progeny, and polymorphic information content are listed in Table 1. The average number of alleles per locus was 4.3 in blue catfish sires and 9.1 in channel catfish dams. Of the total 1697 progeny genotyped, 251 progeny could not be assigned unambiguously to a single sire, 30 could not be assigned unambiguously to a single dam, and 50 were not assigned due to either poor amplification or inadequate match. A total of 1366 hybrid progeny could be unequivocally assigned to both a single sire and dam but 77 of these were not used in the analysis of harvest weight and carcass yield because of physical deformities, primarily ‘stumpbody’, a compacted body previously described in channel catfish by Smitherman (1970). A

2

2

2

2

2

2

2

2

2

2

hs ¼ 4  σ s =σ s þ σ d þ σ sd þ σ e 2

hD ¼ 4  σ D =σ s þ σ d þ σ sd þ σ e

Table 1 Microsatellite allelic polymorphism in 12 channel catfish (Ictalurus punctatus) dams, 22 blue catfish (Ictalurus furcatus) sires, and 1536 channel catfish × blue catfish hybrid offspring. Panel Locus

1

2

3

GY113J02 IpCG0038 IpCG0071 IpCG0195 IpCG0273 71–75 IpCG0148 GY047K03 IpCG0031 IpCG0054 POMC 68–19 IpCG0001 IpCG0032 IpCG0035 IpCG0128

Blue catfish alleles

Channel catfish alleles

Offspring alleles

Non-exclusion

No.

(Parent pair)

No.

Range (bp)

PIC

No.

Range (bp)

PIC

4 3 3 5 3 2 6 4 8 7 2 5 3 4 5 4

386–392 153–169 122–225 227–247 137–145 116–121 206–232 153–169 268–371 414–443 325–328 92–108 162–168 315–341 252–273 262–274

0.624 0.220 0.366 0.430 0.222 0.373 0.764 0.524 0.733 0.718 0.282 0.540 0.408 0.440 0.617 0.495

8 7 7 6 6 11 8 10 10 15 8 9 10 8 12 10

351–400 107–153 198–298 229–247 148–181 113–156 185–218 165–225 275–371 270–346 342–365 82–123 159–186 269–321 261–342 254–324

0.805 10 0.787 0.691 9 0.686 0.773 9 0.783 0.705 8 0.682 0.735 9 0.723 0.870 12 0.813 0.759 12 0.849 0.855 14 0.854 0.867 18 0.886 0.911 22 0.907 0.787 10 0.757 0.765 11 0.759 0.858 10 0.800 0.742 12 0.780 0.871 13 0.811 0.847 10 0.786 Combined non-exclusion probability

PIC — polymorphic information content.

PIC

0.175 0.287 0.184 0.317 0.242 0.149 0.111 0.098 0.069 0.048 0.201 0.194 0.164 0.180 0.151 0.170 1.11e−13

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total of 1288 hybrid progeny were used for analysis of harvest weight and 1101 of those (N 400 g) were used for analysis of carcass yield. A total of 118 full-sib families within 12 maternal half-sib families and 22 paternal half sib families were used in the analysis of harvest weight (Table 2). Four of the 118 full-sib families did not have any progeny N than 400 g and were not used in the analysis of carcass yield. Sire half-sib family size averaged 59 for weight (range 11 to 99) and 50 for carcass yield (range 11 to 82). Dam half-sib family size averaged 107 for weight (range 15 to 220) and 92 for carcass yield (range 6 to 202). Harvest weight averaged 639.4 g (S.D. 231.5) and carcass yield averaged 68.8% (S.D. 1.8). Males were larger than females at harvest, but females had higher carcass yield than males (Table 3). Fish processed on the first day from factorial 2 had higher carcass yield than those processed on the second day from factorial 2. Fish weight had a positive linear relationship with carcass yield (carcass yield increased 0.002% for each 1 g increase in body weight). None of the fixed effects in the model had an effect on the incidence of deformities (overall mean = 5.6%). Variances were highest for dam, intermediate for sire, and lowest for sire × dam interactions for carcass yield and harvest weight (Table 4). Estimates of σ2GCAD (equivalent to dam variance), σ2GCAS (equivalent to sire variance), h2d and h2s (Table 4) were significantly greater than zero for growth and carcass yield, but dam associated variances (σ2GCAD and h2d) were larger than sire variances (σ2GCAS and h2s ) for both traits (~2 fold higher for carcass yield and 10 fold larger for growth). σ2SCA (equivalent to sire × dam interaction variance) was not significantly different for zero for harvest weight or carcass yield. Correlations between BLUP estimates of harvest weight and carcass yield GCA were not significant for either sires (r = 0.27, P = 0.26) or dams (r = −0.25, P = 0.40). Dam explained 7%, sire × dam explained 3% and sire explained 0% of the overall phenotypic variance for incidence of deformities. None of these variance components were significant and therefore, no estimates of genetic effects were made for incidence of deformities.

Table 3 Least square means for fixed effects of sex, factorial, day within factorial for carcass yield and harvest weight of channel catfish (Ictalurus punctatus) female × blue catfish (I. furcatus) male F1 hybrids. Fixed effect

Carcass yield %

Harvest weight (g)

Sex F M SE P-value

69.4 68.7 0.09 b0.0001

568.3 609.4 11.0 0.0002

Factorial 1 2 SE P-value

69.0 69.0 0.60 0.97

666.5 511.2 84.8 0.10

Day (factorial 2) 1 2 SE P-value

69.4 68.7 0.13 b0.0001

– –

et al., 2004), although the differences between genders tend to be smaller for blue catfish than for channel or hybrid catfish (Bosworth, 2012). Smitherman (1970) reported a variety of deformities in channel catfish, including the ‘stumpbody’ phenotype that comprised the majority of deformities observed in this study. The level of deformities observed in hybrid catfish in this study (5.6%) was slightly higher than levels of deformities typically observed in channel catfish (0 to 4.6%) (Bondari and Dunham, 1987; Lovell, 1973). Dunham et al. (1991) demonstrated the stumpbody phenotype was not heritable in channel catfish. The small effects of dam and sire for incidence of deformities in hybrid catfish indicate that there was little genetic basis for the observed deformities. We have previously observed levels of deformities in hybrid catfish similar to those observed in this study, however the incidence is sporadic and appears to be an artifact of hormone induced spawning or the hatching environment rather than the result of genetic abnormalities. Increased deformities associated with hormone induced spawning have been reported in rainbow trout, Oncorhynchus mykiss (Bonnet et al., 2007) and walking catfish, Clarias batrachus (Sahoo et al., 2007). Legendre et al. (2000) observed that an increased time delay between hormone induced ovulation and subsequent stripping and fertilization of eggs resulted in increased deformities in larval striped catfish (Pangasius hypothalamus). We have observed that egg masses from

4. Discussion The pattern of sexually dimorphic growth and carcass yield observed in hybrid catfish in this study (males grow fast than females, females have higher carcass yield than males) has been reported previously for hybrid catfish (Argue et al., 2003; Bosworth, 2012). Similar effects of gender on growth and carcass yield observed in hybrids are also reported for channel and blue catfish (Argue et al., 2003; Bosworth

Table 2 Progeny numbers for carcass yield/harvesta weight of channel catfish (Ictalurus punctatus) female × blue catfish (I. furcatus) male hybrid offspring from 2 factorial matings. Factorial 1

Sire factorial 1

Dam factorial 1

1

2

3

4

5

6

7

8

9

10

11

12

1 2 3 4 5 Sire total factorial 1

1/13 15/15 15/15 0/1 10/11 41/55

3/11 18/19 27/29 1/1 10/15 59/75

3/6 20/21 30/30 2/2 22/22 77/81

5/9 18/19 13/13 1/2 11/11 48/54

1/6 18/19 19/19 ./. 6/7 44/51

1/1 2/2 3/3 ./. 5/5 11/11

./. 9/9 6/6 ./. 12/15 27/30

6/14 28/30 20/22 2/4 24/29 80/99

2/12 9/9 22/30 0/5 10/17 43/73

0/3 7/8 11/12 ./. 9/10 27/33

0/5 13/14 20/23 ./. 6/7 39/49

1/11 18/18 16/18 ./. 7/8 42/55

Factorial 2

Sire factorial 2

Dam factorial 2

13

14

15

16

17

18

19

20

21

22

6 7 8 9 10 11 12 Sire total factorial 2

4/5 ./. 2/3 4/4 1/1 14/14 1/1 26/28

15/15 19/20 14/17 17/19 16/21 ./. 1/1 82/93

14/14 14/14 6/12 12/13 5/7 11/12 4/4 66/76

15/15 10/10 12/14 16/16 5/6 7/7 2/2 67/71

12/13 16/17 9/10 21/22 5/9 6/6 4/4 73/81

7/7 3/3 2/5 10/12 4/5 ./. 2/3 28/35

15/16 14/14 9/12 12/13 2/2 7/7 9/10 68/74

6/6 17/17 7/10 12/13 9/11 14/15 5/5 70/77

3/3 4/4 7/7 3/3 6/7 ./. 2/2 25/26

10/12 12/14 14/14 16/16 2/2 4/4 ./. 58/62 Overall total

a

Dam total factorial 1

23/91 175/183 202/220 6/15 132/157 Total factorial 1 538/666 Dam total factorial 2

Number of progeny measured for carcass yield above the diagonal, number of progeny measured for harvest weight below the diagonal.

101/106 109/113 82/104 123/131 55/71 63/65 30/32 Total factorial 2 563/623 1101/1289

B. Bosworth, G. Waldbieser / Aquaculture 420–421 (2014) 147–153 Table 4 Estimates for variance of dam general combining ability (σ2GCAd), sire general combining ability (σ2GCAs), specific combining ability (σ2SCA), σ2error, heritability for dam half-sib families (h2d), and heritability for sire half-sib families (h2s ) for carcass yield and harvest weight of channel catfish (Ictalurus punctatus) × blue catfish (I. furcatus) F1 hybrid progeny. Genetic parameter estimates

σ2GCAd (± SE) σ2GCAs (± SE) σ2SCA (± SE) σ2error h2d (± SE) h2s (± SE)

Trait Carcass yield

Harvest weight

0.81 (0.39) 0.28 (0.12) 0.07 (0.05) 2.14 0.98 (0.26) 0.34 (0.08)

19,498 (8000) 1608 (778) 833 (615) 38,062 1.30 (0.35) 0.12 (0.06)

hormone induced channel catfish females generally have lower fertility than natural spawns, resulting in increased bacterial and fungal degradation of the egg mass, early hatch of embryos, and increased deformities. Bondari (1983) also reported higher levels of deformities in channel catfish from spawns with reduced hatch rates. Deformed fish were not used in the analysis because the deformities affect the traits being measured and there appears to be no genetic basis for the deformities. The ability to assign progeny to a single channel catfish dam (98.2%) was similar to values for assignment of progeny to a single parent or parent pair (≥95%) using microsatellites in striped bass × white bass hybrids (Wang et al., 2006), common carp, Cyprinus carpio (Vandeputte et al., 2004), and sea bass, Dicentrarchus labrax (Saillant et al., 2006). However, assignment of progeny to a single blue catfish sire was only about 85% and was due to the lower number of alleles in blue sires compared to channel dams. Some blues sires shared at least one allele at all loci which prevented assignment of some progeny to a single sire. The lower level of polymorphism observed for blue catfish relative to channel catfish in this study may be associated with the breeding history of the fish or an artifact of the microsatellite loci used in this study. The blue catfish used in this study were from a single strain of fish obtained from a commercial farm and although their breeding history was not well documented, the source of the fish indicated that the population was derived from a maximum of five to ten full-sib families. The female channel catfish were a mixture of fish from different strains from commercial farms with large breeding populations and therefore would be expected to be more polymorphic. In addition, the microsatellite loci used were originally selected for parentage assignment in channel catfish, and therefore were selected from a large number of loci specifically because they were highly polymorphic in channel catfish (Waldbieser and Bosworth, 2013). We are currently screening blue catfish microsatellite loci to determine if we can identify loci with higher levels of polymorphism that would be more useful for parentage assignment with blue catfish. The magnitude of variance components for growth and carcass yield (and corresponding estimates of combining abilities and heritabilities) was largest for dams, intermediate for sires, and smallest for the sire × dam interactions. The small σ2SCA (equivalent to sire × dam interaction) relative to the larger σ2GCAD and σ2GCAS indicates that the majority of phenotypic variation in carcass yield and growth of F1 hybrid catfish was due to additive genetic effects. Analysis of factorial matings with other fish species also revealed greater degrees of additive variance relative to dominance variance in hybrid striped bass (Wang et al., 2006), sea bass (Saillant et al., 2006), and carp (Vandeputte et al., 2004). We observed that variance component estimates related to dams (σ2GCAD and h2d) were larger than corresponding sire associated variances (σ2GCAS and h2s ). In the mating design used (a factorial with one species used as dam and one as sire) maternal environmental, maternal genetic and additive genetic effects are confounded. The confounding of these factors likely resulted in the high h2d (0.98 and 1.30 for carcass yield and weight, respectively) since heritability estimates have

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a theoretical maximum of 1.0 (all phenotypic variance explained by genetic variance). The larger variance associated with dams (and dam estimates of heritability) compared to sires could be a ‘true’ maternal effect, an ‘induced’ maternal effect associated with hormone induced spawning, or differences in the additive genetic variance in the blue catfish and channel catfish populations used in the study. Maternal effects can be environmental or genetic and reflect differences in either environment or genetic merit of the dam on her progeny's performance (Wilham, 1980). In fish, maternal effects are generally limited to size or nutrient content of the egg. The magnitude of maternal effects typically decline as progeny age, as reported for maternal effects on growth of brown trout, Salmo trutta fario (Vandeputte et al., 2002), rainbow trout (Henryon et al., 2002), hybrid striped bass (Wang et al., 2006) and Atlantic cod, Gadus morhua (Tosh et al., 2010). In contrast, the large dam variances for weight and carcass yield in hybrid catfish reported in this study were present at 520 days posthatch. A long-lasting maternal effect for harvest weight due to differences in size or nutritional composition of eggs is possible; however, we did not observe a maternal effect for harvest size in purebred channel catfish of approximately the same age as the hybrid catfish in this study (unpublished data). An early size advantage could provide an initial competitive advantage to fry which might manifest in a long term growth advantage, especially in a communal rearing environment like the one used in this study. We did not measure egg or larval size in this study so it was not possible to determine if egg or larval size differences affected harvest weight. Hormone-induced ovulation of female channel catfish may have resulted in a ‘human-induced’ maternal effect. Although female channel catfish are selected for hormone-induced ovulation based on external phenotypic signs indicative of reproductive readiness (Dunham and Masser, 2012), it not known how well these indicators correlate with a female's true ‘readiness’ for spawning. Recent research has shown that development and maturation of the teleost egg is a complex process involving multiple coordinated changes in hormones, nutritional factors and maternally derived mRNAs (reviewed by Lubzens et al., 2010). Inducing ovulation in a female a week or two prior to when she would have spawned normally may have an impact on the ‘readiness’ of an egg. An egg that was induced to ovulate prematurely may allow production of a viable progeny but could have an effect on progeny growth or other performance traits. Increased incidence of deformities resulting from hormone-induced ovulation in teleosts was cited earlier, and therefore it is also possible hormone-induced ovulation could have a long-term effect on growth or carcass traits of fish. As mentioned previously, the channel catfish dams were a mixture of fish from three populations and the blue catfish sires were from a single strain that had been through a population bottleneck. Therefore, the greater dam associated variances may simply reflect differences in the additive genetic variance between the channel catfish and blue catfish populations. The microsatellite data presented in this study support the conclusion that the channel catfish dams were more genetically diverse than the blue catfish sires. All of the factors listed (maternal effects, effects related to hormone induced ovulation, and more genetic variation in the channel dams than blue sires) may have contributed to the larger magnitude of variance associated with dams compared to sires. Understanding the basis for the greater variance of dam on hybrid catfish growth and carcass yield will be important in designing breeding programs and broodfish management protocols for hybrid catfish production. The information presented provides perspective on development of breeding programs for genetic improvement of hybrid catfish, and perhaps more importantly, highlights the additional information needed to clearly define the optimal breeding strategy for improving hybrid catfish. The data indicate the presence of substantial additive genetic variation for growth and carcass yield in F1 catfish hybrids, suggesting that sires and dams can be selected on additive genetic merit (i.e. GCA). The greater magnitude of GCA relative to SCA suggests that

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sire and dam effects on progeny performance will be consistent across matings and there will be little benefit from searching for specific, superior sire × dam combinations. Selecting based on GCA should allow fairly rapid improvement in performance by screening and selection of existing sires and dams. In particular, since a relatively small number of sires are needed and little selection or evaluation of blue catfish has been done, fairly intense selection of blue sires could be used to rapidly improve hybrid progeny performance. Cryopreservation of blue sire sperm will play a critical role in breeding plans for improving hybrid catfish performance because males must be sacrificed to obtain sperm. After initial screening and selection of superior purebred parents from existing stocks, an efficient approach for continued improvement of hybrid performance is needed. Crossbred or hybrid genetic improvement programs for crops and livestock typically include some variation of recurrent selection (selection of purebred parents based on purebred performance), reciprocal recurrent selection (selection of purebred parents based on hybrid performance), or development of a synthetic (hybrid offspring used as parents and selection continues in the hybrid population). Development of a synthetic ‘breed’ by continued selection from a F1 hybrid base population eliminates the need to maintain and select within two pure-species populations. However, blue catfish and channel catfish diverged evolutionarily approximately 17 million years ago (Hardman and Hardman, 2008) and substantial genome rearrangements between the species and development of favorable epistatic interactions within each species are probable. Development of an improved synthetic would be hindered by issues associated with genome rearrangements between species and breakdown of favorable epistatic interactions in the synthetic. Dunham and Argue (2000) reported reduced reproductive performance in F2 catfish hybrids associated with loss of favorable epistatic interactions (recombination loss). In addition, heterosis associated with dominance or overdominance genetic effects in the F1 would be reduced over time in the synthetic. The F1 hybrid also offers greater protection of the breeder's investment in germplasm development compared to a synthetic. Currently, the majority of hybrid fingerlings are produced by a small number of specialized hatcheries, who in turn sell those fingerlings to a large number of independent growers who grow them to market-size. Sale of an F1 allows the breeder to retain their improved pure-species germplasm whereas sale of an improved synthetic would transfer the breeders improved germplasm directly to the foodfish grower. Given the potential genetic issues and lack of protection of breeder investment associated with synthetic development, selection for improved F1 hybrid performance through recurrent or reciprocal recurrent selection seems the most likely path for catfish breeders. The relative merits of recurrent selection (selection based on purebred performance) and reciprocal recurrent selection (selection based on crossbred/hybrid performance) depend on the heritability of the trait in purebred and crossbred populations, the correlation between genetic effects for traits of interest in the purebred and hybrid populations and the relative resources (facilities, time, labor etc.) for maintaining and measuring traits each approach requires. Theory developed for improvement of crossbred livestock and hybrid trees indicates that genetic progress can be maximized by a modified reciprocal recurrent selection in which purebreds are selected on an index of purebred and crossbred offspring performance (Kerr et al., 2004; Lo et al., 1997; Wei and van der Werf, 1994). Although the data reported in the current study indicate that the genetic variance for harvest weight and carcass yield in F1 hybrids was primarily additive, it does not necessarily follow that those same additive effects will be present in a purebred population (Gordon, 1999). Theory developed for livestock indicates that the best approach to selecting purebred parents for improved crossbred progeny performance is to consider a single trait (such as growth) as two separate traits in purebred and crossbred populations, determine heritabilities and genetic correlations between the ‘two’ traits, and then select purebred parents on a selection index based on purebred and crossbred

performance (Lo et al., 1997). Wei and van der Werf (1994) demonstrated that selection based on an index of purebred and crossbred performance would be superior to either selection based only on purebred performance or only on crossbred performance, unless the genetic correlation between purebred and crossbred performance was near 1 or 0. If the correlation between purebred and hybrid performance is zero then selection of parents should be based on hybrid performance because purebred performance is not predictive of hybrid performance. If the correlation is 1 then selection could be based on purebred performance because purebred performance is highly predictive of hybrid performance and the extra time and facilities devoted to recording data for both purebred and crossbred progeny are not beneficial. However, in the majority of studies with crossbred livestock, the genetic correlation between the same trait in the purebred and crossbred populations is generally positive, but intermediate in value (Habier et al., 2007; Nakavisut et al., 2005; Newman et al., 2002; Wei and van der Werf, 1995). Therefore, in theory the most efficient selection is based on combined purebred and crossbred performance. Collecting data on both hybrid and purebred populations increases costs and facilities required and generally if the genetic variance is primarily additive and genetic correlation between purebred and hybrid performance is N0.70 selection based only on purebred performance is probably best from a practical perspective. The breeder does have the ability to easily produce large numbers of both purebred and hybrid offspring populations from sires and dams simultaneously with most fish species, an advantage that is not possible or requires technological manipulations (embryo transfer etc.) in many livestock species. We are currently conducting trials to determine the heritabilities and genetic correlations for growth and carcass yield in purebred and hybrid catfish populations and will use this information to determine the ‘best’ strategy for genetic improvement of F1 hybrid catfish performance. 5. Conclusion The data presented indicate the presence of additive genetic variation for growth and carcass yield in hybrid catfish, and that the variation contributed by the channel catfish dams was larger than that of blue catfish sires. The greater variation in hybrid progeny attributed to the channel dams may represent confounding of additive genetic effects with maternal genetic or environmental effects, or greater genetic variation in the channel catfish females used relative to the blue catfish sires used. Additional information on the magnitude of the genetic correlation between hybrid and purebred performance is needed to determine the most efficient breeding program to improve hybrid catfish performance.

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