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Cytology of Fusarium culmorum
[ 225 ] Trans. Br, mycol. Soc. 58 (2), 225-230 (1972) Printed in Great Britain
CYTOLOGY OF FUSARIUM CULMORUM By E. PUNITHALINGAM Commonwealth Mycological Institute, Kew (With Plates 23-26) The distribution, process of division and the behaviour of nuclei during conidium germination were studied in two Fusarium culmorum isolates by the HCI-Giemsa technique. Individual cells of ungerminated conidia were initially uninucleate with the nucleus occupying a medial position. In germinating conidia nuclear division often occurred after the emergence of a phialide or germ-tube. The first nuclear division within the cell of a germinating conidium was followed by the migration of one daughter nucleus into the germ-tube or phialide and the formation of a cross-septum at the base. Subtequently all nuclei formed within the conidia produced by the phialide or within the germtube were derived from the single migratory nucleus. During somatic mitosis chromosomes and spindle apparatus complete with centrioles were observed. Within the conidia and mycelium the haploid chromosome number was eight and this was constant. Mature hyphal cells were often uninucleate but young hyphal apices with 1 -3 nuclei linearly arranged before the laying down of the cross-septa were sometimes observed.
Fusarium culmorum represents a relatively stable and uniform Fusarium species existing saprophytically and parasitically on cereal crops and a wide range of plant species. Although the dynamics of spore swelling, germ-tube initiation and fine structure of this species have received a great deal of attention (Garcia Acha, Aguirre, Uruburu & Villanueva, 1966; Marchant & White, 1965) its cytology has not received extensive investigation. During a study on the fine structure of F. culmorum conidia Garcia Acha et al. (1966) reported that a nucleus was not often observed within the end cells of the conidia nor were structures that might be construed as chromosomal in nature. It would appear from their report that conidial cells of F. culmorum are unlike those of other fungi with multicellular conidia where generally each conidial cell is normally nucleate (Hartmann, 1966; Hrushovetz, 1956). The present study was therefore primarily designed to investigate the distribution of nuclei within cells of ungerminated conidia and mycelium and also their behaviour during conidium germination. MATERIALS AND METHODS
The strains ofF. culmorum used in this study were isolated from Triticum (IMI 125119) and Solanum tuberosum (IMI 122415) and they are in all characters typical F. culmorum isolates. Single spore cultures of the two strains were grown on potato sucrose agar at 24-25 °C and exposed to
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fluorescent light. For staining, conidia were harvested from i -e-week-old cultures. Observations on nuclei within ungerminated and germinated conidia and mycelium were made by the HCl-Giemsa technique and the stages in the staining technique were essentially the same as those described previously by Punithalingam (1970). For investigating the behaviour of nuclei during germination, 0'5 ml of a suspension offreshly harvested conidia in o·5 % dextrose solution was spread in a thin film on clean glass slides and incubated at 25°. Subsequently the slides were removed from the incubation chamber, air dried, fixed in a 3: I ethyl alcohol: acetic acid mixture and stained by the method similar to that employed for mycelium and ungerminated conidia. Stained preparations were air dried and mounted in euparal. DISTRIBUTION OF NUCLEI IN UNGERMINATED CONIDIA
In freshly harvested conidia a nucleus was present within every individual cell including the end cells (PI. 23, fig. I). Individual conidial cells (PI. 23, fig. 2) with more than one nucleus were observed in 5 % of the samples. No conidium was observed with all its individual cells multinucleate. Within the individual conidial cells the nucleus was usually centrally located with the exception of the end cells where the nucleus was generally located close to the septum delimiting the end cell from the penultimate cell. The nuclei in ungerminated conidia were sphaeroid, densely but uniformly stained and consisted of a coarsely granular chromatin matrix during the interdivisional or resting stage. Examination of several interdivisional nuclei revealed an unstained area (PI. 25, fig. 16) indicating the position of the nucleolus (Finley, 1970) which was difficult to recognize by the end of prophase. No nuclear division was observed in freshly harvested ungerminated conidia. BEHAVIOUR OF NUCLEI DURING CONIDIUM GERMINATION
When conidia were incubated they showed considerable swelling similar to that reported by Marchant & White (1965) but no nuclear division occurred during this period. The first nuclear division in incubated conidia (PI. 23, fig. 3; PI. 24, figs. 8, 13) was noticed after the formation of germ-tube or phialide. The central conidial cells on germination often formed a phialide while the end cells generally produced germ-tubes (PI. 24, figs. 8, I I). It was usual to find a single phialide developing from one of the central cells (PI. 23, figs. 5-7; PI. 24, fig. 12; PI. 25, fig. 15) but occasionally two such phialides were formed from one conidium (PI. 25, fig. 17). Subsequent to the formation of phialide or germ-tube the conidial cell nucleus migrated close to the base ofthe phialide or germ-tube and divided once. Following the first nuclear division one daughter nucleus migrated into the phialide or germ-tube (PI. 23, figs. 4,5; PI. 25, fig. 15), while the other sister nucleus moved back to a position originally occupied by the parent nucleus. Nuclear migration was generally followed by the laying down of a septum at the level of the conidial wall and close to the base of the phialide or germ-tube. Only one nucleus was observed to migrate into
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the phialide or germ-tube from the germinating conidium and there was no evidence of further migration of nuclei. From this single migratory nucleus (PI. 23, fig. 4; PI. 24, figs. 9, 14; PI. 25, fig. 17) within the phialide were later derived all the nuclei within the conidia produced by the phialide. BEHAVIOUR OF NUCLEI DURING CONIDIUM FORMATION FROM GERMINATING CONIDIA
The events which resulted in the formation of conidia from a germinating conidium were as follows. The short phialide after having reached 10 pm or more in length developed a bulge at the apex (PI. 23, fig. 5) which eventually grew out into a conidium. With the appearance of the conidial initial the single nucleus within the phialide divided once and one daughter nucleus (PI. 23, fig. 7) migrated into the conidial initial or primordium. This was quickly followed by the laying down of a septum separating the phialide from the conidial primordium with a single nucleus. Subsequently the single nucleus within the conidial primordium migrated to the mid-region before dividing. Nuclear division within the young developing conidium was followed by the laying down of septa resulting in a multicellular conidium with uninucleate individual cells (PI. 24, fig. 10; PI. 25, figs. 16-18). There was, however, no evidence of migration of nuclei through the septum. After the liberation of the first conidium a succession of conidia were produced from the open cylindrical phialide. The behaviour of nuclei during successive conidium formation was essentially similar to the events that took place during the formation of the first conidium. No anastomoses between germ-tubes were observed as often reported for other Fusarium species (Buxton, 1954). Occasionally, germinating conidia, instead of producing a simple short phialide, produced a short hypha from which 2-3 hyphal branches were formed terminating in uninucleate phialides. DISTRIBUTION OF NUCLEI WITHIN MYCELIUM
In general, mature hyphal cells were uninucleate (PI. 26, fig. 2 I) but occasionally during hyphal cell multiplication bi- or trinucleate hyphal cells with the nuclei arranged linearly along the entire length of the hyphae were observed. NUCLEAR DIVISION
Within the conidial and young hyphal cells, nuclei at various stages of division were observed and the details of the division process were essentially the same in both cell structures. The first indication of nuclear division was an increase in size accompanied by the assumption of a slightly irregular shape and the appearance of a reticulate network. This was followed by the appearance of a small, deeply stained body (the centriole) close to the nucleus (PI. 24, fig. 10). Similar structures have been reported in Penicillium griseofulvum (Fletcher, 1969), Ceratobasidium fiavescens and Pellicularia koleroga (Finley, 1970). No direct evidence of the existence of a nucleolus could be found with the staining technique I)
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employed here, but in several nuclei examined an unstained area (PI. 25, fig. 16) within the nucleus was evident, indicating the position of the nucleolus. This unstained area became less distinct as nuclear division progressed. As the division process progressed to a stage that could be regarded as late prophase eight distinct chromosome strands were observed (PI. 26, figs. 19, 20, 22). By this time the centriole had undergone division and a centriolar body was observed at each pole (PI. 26, fig. 19). A spindle apparatus (PI. 24, fig. 14; PI. 26, figs. 19, 23) was evident in several of the preparations examined and it was not restricted to anyone plane of division with relation to the long axis of the cell. At stages corresponding to anaphase chromosomes were deeply stained and more condensed (PI. 26, figs. 22, 23). At metaphase within conidial, germ-tube and mycelial nuclei the number of chromosomes observed was constant. No actual longitudinal splitting of chromosomes was observed. However, chromosome bridges were common during anaphase. By the end of anaphase daughter chromosomes were not recognizable as distinct separate strands. At telophase the daughter nuclei were deeply stained and no further details could be observed. DISCUSSION
The observation that 95 % or more of the conidial cells are uninucleate and that 5 % are bi- or trinucleate does not lend support even in part to the claim made by Garcia Acha et al. (1966). This investigation also establishes that all conidial cells are nucleate including the end cells which are always uninucleate. The evidence that nuclear division was delayed until a germ-tube or phialide had formed is further indication of the initial uninucleate nature of the conidial cells. Although Garcia Acha et al. made a generalized statement regarding the number and distribution of nuclei within conidial cells they failed to state what percentage of the conidia had cells with more than one nucleus, which in fact would have been a more accurate method of expressing distribution of nuclei within conidia. Their failure to observe nuclei within the end cells may perhaps be attributable to their unsuitable staining technique. The differentiation, emergence and movement of chromosomes towards the poles suggest that nuclear division in F. culmorum follows the same pattern as in higher plants. The presence of a centriole at each pole during nuclear division indicated that they may possibly be involved in nuclear division. Electron microscope investigation of somatic nuclear division in Albugo candida (Berlin & Bowen, 1964), Saccharomyces cereuisiae (Robinow & Marak, 1966), Blastocladiella emersonii (Lessie & Lovett, 1968) and Saprolegnia ferax (Heath & Greenwood, 1968) have shown centrioles to be involved. The regular appearance of a constant number of distinct and separate chromatin strands during nuclear division within conidial and mycelial nuclei allows one to assume that they are chromosomes. However, Garda Acha et al. were unable to observe chromosomes and a possible explanation for this is that either they were dealing with too few sections or their investigations were strictly confined to nuclei in the interdivisional state.
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I wish to thank Mr D. W. Fry for his help with the photography and Mr A. Johnston, Dr C. Booth and Mrs G. Butterfill for reading my typescript. REFERENCES
BERLIN, J. D. & BOWEN, C. C. (1964). Centrioles in the fungus Albugo candida. American Journal of Botany 51, 650-652. BUXTON, E. W. (1954)' Heterocaryosis and variability in Fusarium oxysporum f. gladioli Snyder & Hansen. Journal of General Microbiology 10,71-84, FINLEY, D. E. (1970). Somatic mitosis in Ceratobasidiumflavescens and Pellicularia koleroga, Mycologia 62, 474-485. FLETCHER, J. (1969). Morphology and nuclear behaviour of germinating conidia of Penicillium griseofulvum. Transactions of the British Mycological Society 53, 425-432. GARCiA ACHA, I., AGUIRRE, M.J. R., URUBURU, F. & VILLANUEVA,]. R. (1966). The fine structure of the Fusarium culmorum conidium. Transactions of the British Mycological Society 49, 695-702. HARTMANN, G. C. (1966). The cytology of Alternaria tenuis. Mycologia 58, 694-701. HEATH, I. B. & GREENWOOD, A. D. (1968). Electron microscopic observations of dividing somatic nuclei of Saprolegnia. Journal oj General Microbiology 53, 287-289. HRUSHOVETZ, S. B. (1956). Cytological studies of Helminihosporium sativum. Canadian Journal oj Botany 34,321-327. LESSIE, P. E. & LOVETT, J. S. (1968). Ultrastructural changes during sporangium formation and zoospore differentiation in Blastocladiella emersonii. American Journal oj Botany 55, 220-236. MARCHANT, R. & WHITE, M. F. (1965). Spore swelling and germination in Fusarium culmorum. Journal of General Microbiology 42, 237-244. PUNITHALINGAM, E. (1970). Studies on Sphaeropsidales in culture. Mycological Papers 119, 1 -24. ROBINOW, C. F. & MARAK, J. (1966). A fiber apparatus in the nucleus of the yeast cell. Journal of Cell Biology 29, 129-151.
EXPLANATION OF PLATES
HCI-Giemsa preparations of Fusarium culmorum PLATE 23
All figures x 2000 Fig. I. Ungerminated conidia with uninucleate cells. Fig. 2. An ungerminated conidium with one of its cells binucleate. Fig. 3. A germinated conidium with dividing nucleus at telophase (t) within germinated cell. Fig. 4. A germinated conidium with a phialide and migratory nucleus (m). Fig. 5. A germinated conidium with a phialide (p), migratory nucleus (m) and conidial primordium (cp). Fig. 6. A germinated conidium with migratory nucleus (m) located at the neck of the phialide (P) before division. A conidial primordium (cp) without a nucleus. Note the appearance of centrioles (c) associated with the conidial nuclei. Fig. 7. A germinated conidium with a single nucleus within the conidial primordium and one in the phialide. PLATE
24
All figures x 2000 Fig. 8. A conidium germinating from a central cell and an end cell. Fig. g. A recently formed conidium (s), which has germinated, still attached to the parent spore (pc). The migratory nucleus (m) is at the neck of the germ-tube. Fig. 10. A germinated conidium with its nuclei at different division stages. A centriole (c) is associated with one nucleus.
Transactions British Mycological Society Fig. II. Germination from end cells. A migratory nucleus (m) is close to the germ-tube. Fig. 12. A germinated conidium with phialide and migratory nucleus. Fig. 13. A germinated conidium showing first nuclear division at late telophase (It). Fig. 14. Germinated conidia with migratory nuclei (m) at different divisional stages; (a) anaphase. PLATE 25
All figures x 2000 Fig. 15. A germinated conidium with migratory nucleus within the phialide before division. Fig. 16. A germinated conidium showing secondary conidium formation on germination. Resting nuclei with unstained areas indicating the position of the nucleoli (n). Fig. 17. A germinated conidium with two phialides formed from the same conidial cell. The migratory nucleus in one phialide has not yet divided while the other phialide has already formed a mature spore (s). Fig. 18. A germinated spore with phialide (p) and a mature spore (s) with uninucleate cells. PLATE 26
All figures x 4000 Fig. 19. Mycelial nuclei at different divisional stages. Two nuclei at prophase (pr) with chromatin strands (ch) and a nucleus at anaphase (a) showing centrioles (c) and spindle apparatus (sp). Fig. 20. Nuclear division at late prophase showing centriole (c) and chromatin strands (ch). Fig. 21. Uninucleate hyphal cell. Nucleus at prophase showing centriole (c) and chromatin strands. Fig. 22. Nuclear division within germinating spore; metaphase (me) and anaphase (a) viewed from the equator. Fig. 23. Nuclear division within germinated spore; anaphase (a) and telophase (t).
(Acceptedfor publication 25 February 1971)
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CYTOLOGY OF FUSARIUM CULMORUM By E. PUNITHALINGAM Commonwealth Mycological Institute, Kew (With Plates 23-26) The distribution, process of division and the behaviour of nuclei during conidium germination were studied in two Fusarium culmorum isolates by the HCI-Giemsa technique. Individual cells of ungerminated conidia were initially uninucleate with the nucleus occupying a medial position. In germinating conidia nuclear division often occurred after the emergence of a phialide or germ-tube. The first nuclear division within the cell of a germinating conidium was followed by the migration of one daughter nucleus into the germ-tube or phialide and the formation of a cross-septum at the base. Subtequently all nuclei formed within the conidia produced by the phialide or within the germtube were derived from the single migratory nucleus. During somatic mitosis chromosomes and spindle apparatus complete with centrioles were observed. Within the conidia and mycelium the haploid chromosome number was eight and this was constant. Mature hyphal cells were often uninucleate but young hyphal apices with 1 -3 nuclei linearly arranged before the laying down of the cross-septa were sometimes observed.
Fusarium culmorum represents a relatively stable and uniform Fusarium species existing saprophytically and parasitically on cereal crops and a wide range of plant species. Although the dynamics of spore swelling, germ-tube initiation and fine structure of this species have received a great deal of attention (Garcia Acha, Aguirre, Uruburu & Villanueva, 1966; Marchant & White, 1965) its cytology has not received extensive investigation. During a study on the fine structure of F. culmorum conidia Garcia Acha et al. (1966) reported that a nucleus was not often observed within the end cells of the conidia nor were structures that might be construed as chromosomal in nature. It would appear from their report that conidial cells of F. culmorum are unlike those of other fungi with multicellular conidia where generally each conidial cell is normally nucleate (Hartmann, 1966; Hrushovetz, 1956). The present study was therefore primarily designed to investigate the distribution of nuclei within cells of ungerminated conidia and mycelium and also their behaviour during conidium germination. MATERIALS AND METHODS
The strains ofF. culmorum used in this study were isolated from Triticum (IMI 125119) and Solanum tuberosum (IMI 122415) and they are in all characters typical F. culmorum isolates. Single spore cultures of the two strains were grown on potato sucrose agar at 24-25 °C and exposed to
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fluorescent light. For staining, conidia were harvested from i -e-week-old cultures. Observations on nuclei within ungerminated and germinated conidia and mycelium were made by the HCl-Giemsa technique and the stages in the staining technique were essentially the same as those described previously by Punithalingam (1970). For investigating the behaviour of nuclei during germination, 0'5 ml of a suspension offreshly harvested conidia in o·5 % dextrose solution was spread in a thin film on clean glass slides and incubated at 25°. Subsequently the slides were removed from the incubation chamber, air dried, fixed in a 3: I ethyl alcohol: acetic acid mixture and stained by the method similar to that employed for mycelium and ungerminated conidia. Stained preparations were air dried and mounted in euparal. DISTRIBUTION OF NUCLEI IN UNGERMINATED CONIDIA
In freshly harvested conidia a nucleus was present within every individual cell including the end cells (PI. 23, fig. I). Individual conidial cells (PI. 23, fig. 2) with more than one nucleus were observed in 5 % of the samples. No conidium was observed with all its individual cells multinucleate. Within the individual conidial cells the nucleus was usually centrally located with the exception of the end cells where the nucleus was generally located close to the septum delimiting the end cell from the penultimate cell. The nuclei in ungerminated conidia were sphaeroid, densely but uniformly stained and consisted of a coarsely granular chromatin matrix during the interdivisional or resting stage. Examination of several interdivisional nuclei revealed an unstained area (PI. 25, fig. 16) indicating the position of the nucleolus (Finley, 1970) which was difficult to recognize by the end of prophase. No nuclear division was observed in freshly harvested ungerminated conidia. BEHAVIOUR OF NUCLEI DURING CONIDIUM GERMINATION
When conidia were incubated they showed considerable swelling similar to that reported by Marchant & White (1965) but no nuclear division occurred during this period. The first nuclear division in incubated conidia (PI. 23, fig. 3; PI. 24, figs. 8, 13) was noticed after the formation of germ-tube or phialide. The central conidial cells on germination often formed a phialide while the end cells generally produced germ-tubes (PI. 24, figs. 8, I I). It was usual to find a single phialide developing from one of the central cells (PI. 23, figs. 5-7; PI. 24, fig. 12; PI. 25, fig. 15) but occasionally two such phialides were formed from one conidium (PI. 25, fig. 17). Subsequent to the formation of phialide or germ-tube the conidial cell nucleus migrated close to the base ofthe phialide or germ-tube and divided once. Following the first nuclear division one daughter nucleus migrated into the phialide or germ-tube (PI. 23, figs. 4,5; PI. 25, fig. 15), while the other sister nucleus moved back to a position originally occupied by the parent nucleus. Nuclear migration was generally followed by the laying down of a septum at the level of the conidial wall and close to the base of the phialide or germ-tube. Only one nucleus was observed to migrate into
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the phialide or germ-tube from the germinating conidium and there was no evidence of further migration of nuclei. From this single migratory nucleus (PI. 23, fig. 4; PI. 24, figs. 9, 14; PI. 25, fig. 17) within the phialide were later derived all the nuclei within the conidia produced by the phialide. BEHAVIOUR OF NUCLEI DURING CONIDIUM FORMATION FROM GERMINATING CONIDIA
The events which resulted in the formation of conidia from a germinating conidium were as follows. The short phialide after having reached 10 pm or more in length developed a bulge at the apex (PI. 23, fig. 5) which eventually grew out into a conidium. With the appearance of the conidial initial the single nucleus within the phialide divided once and one daughter nucleus (PI. 23, fig. 7) migrated into the conidial initial or primordium. This was quickly followed by the laying down of a septum separating the phialide from the conidial primordium with a single nucleus. Subsequently the single nucleus within the conidial primordium migrated to the mid-region before dividing. Nuclear division within the young developing conidium was followed by the laying down of septa resulting in a multicellular conidium with uninucleate individual cells (PI. 24, fig. 10; PI. 25, figs. 16-18). There was, however, no evidence of migration of nuclei through the septum. After the liberation of the first conidium a succession of conidia were produced from the open cylindrical phialide. The behaviour of nuclei during successive conidium formation was essentially similar to the events that took place during the formation of the first conidium. No anastomoses between germ-tubes were observed as often reported for other Fusarium species (Buxton, 1954). Occasionally, germinating conidia, instead of producing a simple short phialide, produced a short hypha from which 2-3 hyphal branches were formed terminating in uninucleate phialides. DISTRIBUTION OF NUCLEI WITHIN MYCELIUM
In general, mature hyphal cells were uninucleate (PI. 26, fig. 2 I) but occasionally during hyphal cell multiplication bi- or trinucleate hyphal cells with the nuclei arranged linearly along the entire length of the hyphae were observed. NUCLEAR DIVISION
Within the conidial and young hyphal cells, nuclei at various stages of division were observed and the details of the division process were essentially the same in both cell structures. The first indication of nuclear division was an increase in size accompanied by the assumption of a slightly irregular shape and the appearance of a reticulate network. This was followed by the appearance of a small, deeply stained body (the centriole) close to the nucleus (PI. 24, fig. 10). Similar structures have been reported in Penicillium griseofulvum (Fletcher, 1969), Ceratobasidium fiavescens and Pellicularia koleroga (Finley, 1970). No direct evidence of the existence of a nucleolus could be found with the staining technique I)
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employed here, but in several nuclei examined an unstained area (PI. 25, fig. 16) within the nucleus was evident, indicating the position of the nucleolus. This unstained area became less distinct as nuclear division progressed. As the division process progressed to a stage that could be regarded as late prophase eight distinct chromosome strands were observed (PI. 26, figs. 19, 20, 22). By this time the centriole had undergone division and a centriolar body was observed at each pole (PI. 26, fig. 19). A spindle apparatus (PI. 24, fig. 14; PI. 26, figs. 19, 23) was evident in several of the preparations examined and it was not restricted to anyone plane of division with relation to the long axis of the cell. At stages corresponding to anaphase chromosomes were deeply stained and more condensed (PI. 26, figs. 22, 23). At metaphase within conidial, germ-tube and mycelial nuclei the number of chromosomes observed was constant. No actual longitudinal splitting of chromosomes was observed. However, chromosome bridges were common during anaphase. By the end of anaphase daughter chromosomes were not recognizable as distinct separate strands. At telophase the daughter nuclei were deeply stained and no further details could be observed. DISCUSSION
The observation that 95 % or more of the conidial cells are uninucleate and that 5 % are bi- or trinucleate does not lend support even in part to the claim made by Garcia Acha et al. (1966). This investigation also establishes that all conidial cells are nucleate including the end cells which are always uninucleate. The evidence that nuclear division was delayed until a germ-tube or phialide had formed is further indication of the initial uninucleate nature of the conidial cells. Although Garcia Acha et al. made a generalized statement regarding the number and distribution of nuclei within conidial cells they failed to state what percentage of the conidia had cells with more than one nucleus, which in fact would have been a more accurate method of expressing distribution of nuclei within conidia. Their failure to observe nuclei within the end cells may perhaps be attributable to their unsuitable staining technique. The differentiation, emergence and movement of chromosomes towards the poles suggest that nuclear division in F. culmorum follows the same pattern as in higher plants. The presence of a centriole at each pole during nuclear division indicated that they may possibly be involved in nuclear division. Electron microscope investigation of somatic nuclear division in Albugo candida (Berlin & Bowen, 1964), Saccharomyces cereuisiae (Robinow & Marak, 1966), Blastocladiella emersonii (Lessie & Lovett, 1968) and Saprolegnia ferax (Heath & Greenwood, 1968) have shown centrioles to be involved. The regular appearance of a constant number of distinct and separate chromatin strands during nuclear division within conidial and mycelial nuclei allows one to assume that they are chromosomes. However, Garda Acha et al. were unable to observe chromosomes and a possible explanation for this is that either they were dealing with too few sections or their investigations were strictly confined to nuclei in the interdivisional state.
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I wish to thank Mr D. W. Fry for his help with the photography and Mr A. Johnston, Dr C. Booth and Mrs G. Butterfill for reading my typescript. REFERENCES
BERLIN, J. D. & BOWEN, C. C. (1964). Centrioles in the fungus Albugo candida. American Journal of Botany 51, 650-652. BUXTON, E. W. (1954)' Heterocaryosis and variability in Fusarium oxysporum f. gladioli Snyder & Hansen. Journal of General Microbiology 10,71-84, FINLEY, D. E. (1970). Somatic mitosis in Ceratobasidiumflavescens and Pellicularia koleroga, Mycologia 62, 474-485. FLETCHER, J. (1969). Morphology and nuclear behaviour of germinating conidia of Penicillium griseofulvum. Transactions of the British Mycological Society 53, 425-432. GARCiA ACHA, I., AGUIRRE, M.J. R., URUBURU, F. & VILLANUEVA,]. R. (1966). The fine structure of the Fusarium culmorum conidium. Transactions of the British Mycological Society 49, 695-702. HARTMANN, G. C. (1966). The cytology of Alternaria tenuis. Mycologia 58, 694-701. HEATH, I. B. & GREENWOOD, A. D. (1968). Electron microscopic observations of dividing somatic nuclei of Saprolegnia. Journal oj General Microbiology 53, 287-289. HRUSHOVETZ, S. B. (1956). Cytological studies of Helminihosporium sativum. Canadian Journal oj Botany 34,321-327. LESSIE, P. E. & LOVETT, J. S. (1968). Ultrastructural changes during sporangium formation and zoospore differentiation in Blastocladiella emersonii. American Journal oj Botany 55, 220-236. MARCHANT, R. & WHITE, M. F. (1965). Spore swelling and germination in Fusarium culmorum. Journal of General Microbiology 42, 237-244. PUNITHALINGAM, E. (1970). Studies on Sphaeropsidales in culture. Mycological Papers 119, 1 -24. ROBINOW, C. F. & MARAK, J. (1966). A fiber apparatus in the nucleus of the yeast cell. Journal of Cell Biology 29, 129-151.
EXPLANATION OF PLATES
HCI-Giemsa preparations of Fusarium culmorum PLATE 23
All figures x 2000 Fig. I. Ungerminated conidia with uninucleate cells. Fig. 2. An ungerminated conidium with one of its cells binucleate. Fig. 3. A germinated conidium with dividing nucleus at telophase (t) within germinated cell. Fig. 4. A germinated conidium with a phialide and migratory nucleus (m). Fig. 5. A germinated conidium with a phialide (p), migratory nucleus (m) and conidial primordium (cp). Fig. 6. A germinated conidium with migratory nucleus (m) located at the neck of the phialide (P) before division. A conidial primordium (cp) without a nucleus. Note the appearance of centrioles (c) associated with the conidial nuclei. Fig. 7. A germinated conidium with a single nucleus within the conidial primordium and one in the phialide. PLATE
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All figures x 2000 Fig. 8. A conidium germinating from a central cell and an end cell. Fig. g. A recently formed conidium (s), which has germinated, still attached to the parent spore (pc). The migratory nucleus (m) is at the neck of the germ-tube. Fig. 10. A germinated conidium with its nuclei at different division stages. A centriole (c) is associated with one nucleus.
Transactions British Mycological Society Fig. II. Germination from end cells. A migratory nucleus (m) is close to the germ-tube. Fig. 12. A germinated conidium with phialide and migratory nucleus. Fig. 13. A germinated conidium showing first nuclear division at late telophase (It). Fig. 14. Germinated conidia with migratory nuclei (m) at different divisional stages; (a) anaphase. PLATE 25
All figures x 2000 Fig. 15. A germinated conidium with migratory nucleus within the phialide before division. Fig. 16. A germinated conidium showing secondary conidium formation on germination. Resting nuclei with unstained areas indicating the position of the nucleoli (n). Fig. 17. A germinated conidium with two phialides formed from the same conidial cell. The migratory nucleus in one phialide has not yet divided while the other phialide has already formed a mature spore (s). Fig. 18. A germinated spore with phialide (p) and a mature spore (s) with uninucleate cells. PLATE 26
All figures x 4000 Fig. 19. Mycelial nuclei at different divisional stages. Two nuclei at prophase (pr) with chromatin strands (ch) and a nucleus at anaphase (a) showing centrioles (c) and spindle apparatus (sp). Fig. 20. Nuclear division at late prophase showing centriole (c) and chromatin strands (ch). Fig. 21. Uninucleate hyphal cell. Nucleus at prophase showing centriole (c) and chromatin strands. Fig. 22. Nuclear division within germinating spore; metaphase (me) and anaphase (a) viewed from the equator. Fig. 23. Nuclear division within germinated spore; anaphase (a) and telophase (t).
(Acceptedfor publication 25 February 1971)
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