Outcrossing rate in adjacent ‘Maya’ and ‘Tommy Atkins’ mango blocks

SCIENTIA HORTlCULTUM ELSEVIER

Scientia Horticulturae 70 (1997) 25-30

Outcrossing rate in adjacent ‘Maya’ and ‘Tommy Atkins’ mango blocks C. Degani a** , 0. Yutko b, R. El-Batsri a, S. Gazit b ’ Institute of Horticulture, Agricultural Research Organization, The Volcani b The Kennedy-Leigh Centre for Horticultural Research, The Hebrew Unillersity

Center, Bet-Dagan. Israel of Jerusalem, Rehotaot, Israel

Accepted 20 March 1997

Abstract Outcrossing rate in mango (Mungifera in&a L.) mature fruit was studied for the first time in an orchard consisting of two adjacent blocks of ‘Maya’ and ‘Tommy Atkins’. Few polyembryonic rootstocks were detected in the blocks. Outcrossing rates were determined at different distances from the pollenizer block using the polymorphic enzyme systems triosephosphate isomerase and isocitrate dehydrogenase. The outcrossing rate was measured in ‘Maya’ for two consecutive years and in ‘Tommy Atkins’ for one year. In all three cases, a pronounced decrease in hybrid rate was found between fruit sampled from the row bordering the second cultivar and that from the most distant row sampled (108 m and 96 m for ‘Maya’ in 1992 and 1993, respectively, and 80 m for ‘Tommy Atkins’). Outcrossing rates were: 63% and 34% for ‘Maya’ in 1992; 5i’% and 39% for ‘Maya’ in 1993, and 79% and 24% for ‘Tommy Atkins’, for fruit sampled from the row closest to and most distant from the pollenizing cultivar, respectively. 0 1997 Elsevier Science B.V. Ke)+vords:

Man&era

indica;

Isozyme analysis; Pollen-parent effect

1. Introduction

Reports differ on the issue of whether mango (Man&era indica L.) is a self-pollinated species.Popenoe(1920) pointed out that mango is self-compatible. Naik and Rao (1943) and Sturrock (1944) also consideredmango flowers to be self-fertile. McGregor (1976) indicated that cross-pollination in mango is not critical, at least for most cultivars. Hand-pollination studies on ‘Haden’ showed that there was no significant difference between the percentages of fruit set in self- vs. cross-pollinated flowers * Corresponding author. Tel.: 972-3-9683396; Fax: 972-3-9669583. 0304-4238/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO304-4238(97)00038-l

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(Young, 1942). On th e other hand, Ruehle and Lynch (1939) found that solid blocks of ‘Haden’ produced very light crops, whereas ‘Haden’ blocks interplanted with seedlings of ‘Saigon’ or ‘Turpentine’ produced good yields, especially on the side of the tree facing the pollenizer. Later however, Ledin (19.58) did not find any advantage in terms of fruit yield by interplanting ‘Haden’ with ‘Saigon’. In a recent study, De Wet et al. (1989) showed that cross-pollination of ‘Haden’ with ‘Zill’ pollen gave a significantly higher percentage of stigmas with germinating pollen grains than self-pollination, suggesting partial self-incompatibility in ‘Haden’. Hand-pollination studies have established the existence of self-incompatibility in several commercial Indian mango cultivars (Mukherjee et al., 1968; Sharma and Singh. 1970, 1972; Singh et al., 1962). Anatomical studies in ‘Dashehari’, ‘Chausa’, ‘Langra’ and ‘Bombay Green’ have shown that although fertilization takes place after selfing, the endosperm degenerates 15 days after pollination and subsequently the selfed fruitlets abscise (Mukherjee et al., 1968; Sharma and Singh, 1970). Mango requires insect pollinators to bring about effective pollen transfer (Mallik. 1957; Free and Williams, 1976; Singh, 1988). Various insect species including flies. beetles, large ants, wasps and bees pollinate mango in different countries (Singh, 1988; Jiron and Hedstrom, 1985; Anderson et al., 1982; Young, 1942; McGregor, 1976). In Israel, flies, bees and beetles are the principal insects visiting mango. The honeybee was found to be an effective pollinator for mango whenever there is no more attractive bloom nearby (Dag and Gazit, 1996). Dolev (1979) found that single trees of the Israeli commercial cultivar ‘Maya’, caged in a screenhouse with a beehive, yielded a heavy crop, similar to that obtained in ‘Maya’ trees subjected to open-pollination. In recent studies, isozyme analysis was used to show that the fruit progeny produced in ‘Maya’ trees caged with a beehive are the product of self-pollination (Degani et al., 1992; Yutko, 1995). These results demonstrate that ‘Maya’ is a self-compatible cultivar. The objective of the present study is to evaluate the response of a ‘Maya’ block planted in close proximity to ‘Tommy Atkins’ to cross-pollination. The effect of ‘Maya’ on outcrossing rate in the ‘Tommy Atkins’ block was also studied.

2. Materials

and methods

2.1. Plant material The study was carried out in adjacent ‘Maya’ and ‘Tommy Atkins’ blocks in the commercial mango orchard of Ramat Magshimim, located in the Lower Golan, Israel. The ‘Tommy Atkins’ block consisted of 19 rows (5 m apart) with 38 to 42 trees per row (3 m apart). The ‘Maya’ block consisted of 18 rows (6 m apart) with 41 to 43 trees per row (3 m apart). In addition, a few seedlings of the polyembryonic rootstocks ‘l-7’, ‘4-9’ and ‘13-l ’ were interspersed in the blocks. Mature fruit was sampled at harvest at four to five distances from the first row of the pollenizer block and not in close proximity to the polyembryonic rootstocks. ‘Maya’ was sampled in 1992 and 1993. whereas ‘Tommy Atkins’ was sampled only in 1993. At each distance, fifty fruits were

C. Degani Table 1 lsozyme genotypes

of the cultivars

Cultivar Maya Tommy l-7 4-9 13-I

Atkins

et al. / Scientia

planted

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70 11997) 25-30

21

orchard

Tpi

Idh

aa bb ab ab ab

cc cc ac ac ac

randomly sampled from ten trees (five per tree) and their seeds were husked and individually planted in 750 ml plastic pots containing peat moss mixed with polystyrene flakes (1: 1 v/v) and a slow-release fertilizer. Seedlings were grown in the greenhouse (28”/ 18°C day/night). 2.2. Isozyme analysis Leaf extracts of the emerged seedlings were analyzed for triosephospate isomerase (TPI; E.C. 5.3.1.1) and isocitrate dehydrogenase (IDH; E.C. 1.1.1.42) according to Degani et al. (1990). Tpi and Idh loci were found to be not linked in several mango cultivars (Aron et al., 1997). The isozyme genotypes of the cultivars planted in the Ramat Magshimim orchard for these polymorphic enzyme systems are shown in Table 1. For Tpi, ‘Maya’ and ‘Tommy Atkins’ were homozygous (aa and bb, respectively). Both cultivars were also homozygous (cc) for Idh. In contrast, the polyembryonic cultivars were heterozygous for both Tpi and Idh. Thus, seedlings heterozygous for Idh were offspring of the polyembryonic cultivars. Since hybrids for Idh are expected to segregate in a homozygote to heterozygote ratio of 1: 1, the actual number of hybrids for Idh should be double the observed number of heterozygous seedlings. The net hybrid percentage of ‘Maya’ X ‘Tommy Atkins’ and ‘Tommy Atkins’ X ‘Maya’, excluding the polyembryonic rootstocks, offspring, was calculated according to the following equation:

N, - N2

Net hybrid percentage = ----x 4 - J-6

100

where N, = number of seedlings heterozygous for Tpi, N2 = number of seedlings heterozygous for both Tpi and Idh, N3 = total number of seedlings analyzed and N4 = calculated number of hybrid seedlings for Idh.

3. Results and discussion Outcrossing rate at increasing distances from the pollenizer block was determined at harvest in adjacent ‘Maya’ and ‘Tommy Atkins’ blocks using the isozyme markers Tpi and Idh. The highest hybrid percentage was always found in the row adjacent to the pollenizer row (Tables 2 and 3). A gradual significant decrease in hybrid rate with

C. Degani

28

Table 2 Outcrossing Distance ‘Tommy

rates in the ‘Maya’ from Atkins’

block

et al. / Scientia

at different

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distances

70 ! 1997) 25-30

from the ‘Tommy

Atkins’

Hybrids

(m)

Seedlings analyzed (no.)

1992 6 30

45 34

9 18

63ab 54 ab

60 108

25 40

0 5

24 b 34 b

12

44 44

5 9

24 48 96

50 47 47

0 4 13

51 a 38 a 52 a 40 a 39 a

block

(%)

Non-‘Tommy pwev

Atkins’

‘Tommy progeny

Atkins’ d

1993 6

’ After subtracting the calculated percentage of non-‘Tommy Atkins’ progeny. b Results within a column followed by a different letter differ significantly by chi-square Bonferroni multiple-comparison test.

test, followed

by

increasing distance from the pollenizer block was found with ‘Tommy Atkins’ (Table 3). In ‘Maya’, the decrease (Table 2); was significant in 1992, but not in 1993 (Table 2). Regardless of whether the pollen source was ‘Maya’ or ‘Tommy Atkins’, substantial outcrossing rates were found at large distances from the pollenizing-cultivar block: in ‘Maya’, 34% at 108 m in 1992 and 39% at 96 m in 1993 (Table 2); in ‘Tommy Atkins’ 24% at 80 m in 1993 (Table 3). These outcrossing rates demonstrate pollen transfer over large distances within the orchard. The distance involved (up to 108 m) is not unusual for bees foraging on mango trees (Wolfenbarger, 1957). It should be remembered, however, that paternity analysis by isozymes measures the ‘net, apparent’ outcrossing rate. The measured ‘net’ outcrossing rate does not only depend on how far the pollen is transferred, but also on the selective advantage it confers on its progeny. Thus, it is

Table 3 Outcrossing Distance

rates in the ‘Tommy from

‘Maya’

(m)

Atkins’ Seedlings

block

at different

distances

from

analyzed

(no.)

Hybrids

(%)

Non’Maya’ 5

43

10

44

20

45

9 9 22

40 80

41 39

10 5

a After subtracting the calculated percentage of non-‘Maya’ progeny. b Results within a column followed by a different letter differ significantly Bonferroni multiple-comparison test.

the ‘Maya’

progeny

block (1993)

‘Maya’ 79ab

51bc 24c

progeny

a

78~ 69 ab

by chi-square

test. followed

by

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possible that the rate of cross-pollination at large distances was much lower than the values found in mature fruit (Tables 2 and 31, but selective abscission of selfed fruitlets due to survivorship advantage of hybrid fruitlets brought about a high rate of hybrid fruit at harvest. Selective abscission of selfed fruitlets has been also demonstrated in avocado (Degani et al., 1989; Vrecenar-Gadus and Ellstrand, 198.5) and lychee (Degani et al., 1995). Flowering behavior in mango does not encourage cross-pollination over self-pollination. Thus, we can assume that self-pollination in a tree will be much more prevalent than pollination with a distant foreign pollen. The predominance of outcrossed fruit in rows adjacent to the pollenizer block (Tables 2 and 3) and the unexpectedly high hybrid percentage at large distances from the pollenizer block could be due to: 1) the existence of self-incompatibility, which is not the case, at least for ‘Maya’ (Dolev, 1979; Degani et al., 1992; Yutko, 19951, or 2) genetic selection favoring outcrossed fruits.

Acknowledgements This research was supported by Research Grant Award No. IS-1651-89 from BARD, The United States-Israel Binational Agricultural Research and Development Fund. The authors wish to thank H. Voet for her valuable help in the statistical analysis. This paper is a contribution from the Agricultural Research Organization, The Volcani Center, Bet-Dagan, Israel, No. 1878-E, 1996 series.

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