Meiosis in diploid and tetraploid desynaptics of pearl millet

Proe. Indian Acad. Sci., Vol. 87 B, No. 2, February 1978, pp. 17-22, I~) Printed in India. Meiosis in diploid and tetraploid desynaptics of pearl millet M V SUBBA RAO Department of Botany, Andhra University, Waltair 530 003 MS received 17 August 1977 Abstract. In the progeny of desynaptic diploids, one desynaptic tetroploid is obtained. A comparative study of the PMC meiosis in the desynaptic diploids and the desynaptic tetraploid is carried out. The desynaptic tetraploid is characterised by high univalent and low multivalent frequency. The mean chiasma frequency in the desynaptic tetraploid is only slightly higher than that in normal diploid plants and nearly twice the maximum frequency observed for the desynaptic diploids. Different possibilities for the origin of the desynaptic tetraploid and its genetic constitution are discussed. Keywords. Pearl millet; desynapsis; tetraploid. 1. Introduction Desynapsis is the mutant condition where chromosomes pair at zygotene and pachytene but fail to remain paired during subsequent stages of meiosis. Occurrence of desynaptic mutants in pearl millet was reported earlier by Krishnaswamy et al (1949), Patil and Vohra (1962), Jauhar (1969), Dhesi et al (1973) and Singh et al (1977). Studies of these authors dealt with desynapsis at diploid level. The present paper deals with a study of meiosis in a desynaptic tetraploid and its comparison to that in desynaptic diploids. 2. Materials and methods In the cultures of desynaptic diploid pearl millet, Pennisetum typhoids (Burm.) Stapf & Hubb., (P. americanum L.), maintained for cytogenetic studies, one desynaptic tetraploid was located in open pollinated progeny. A total of ten desynaptic diploid plants and a single desynaptic tetraploid were used in the present study. All the fixations were made on the same day and almost at the same time. The young ear heads were fixed in 1 : 3 acetic acid; methanol for 24 hr and then transferred to 70 y. methanol and stored in refrigerator till further use. All the photographs were taken from fresh acetocarmine squash preparations. 3. Results In both the tetraploid and diploid plants pachytene stage appeared quite normal without giving evidence of single threads whereas at diakinesis variable number of univalents were observed and hence the plants were considered to be desyaaptie. 17

18 M V Subba Rao The data on pairing behaviour of chromosomes in desynaptic tetraploid and diploid plants were gathered at diakinesis. There were wide differences in the univalent frequency among the different desynaptic diploid plants (figures 1 and 2). The frequency of univalents in the desynaptic diploid plants varied from 8.6 to 97.9 70 whereas in the desynaptic tetraploid the univalent frequency observed was 18"877o. The different types of chromosome associations observed at diakinesis in the desynaptic tetraploid were quadrivalents, trivalents and bivalents. The mean frequencies of different types of chromosome associations at diakinesis were 1-02~o, I-3m, 7"37u and 5.28~ per PMC. Of the different quadrivalent configurations observed, types 11 and 17 (Darlington 1937) were more frequent i.e., 70-670 and 21.6 70 respectively (figure 3). Majority of the trivalents (86.92 70) were of chain type (figure 4). The frequency of rod bivalents (72"970) was higher than that of ring bivalents (27.147o). Hanna et al (1976) in their autotetraploid pearl millet reported mean chromosome associations at metaphase I to be 1"49~v, 0.38,~, 8"47u and 2.64~. By comparing the present data on chromosome associations in the desynaptie tetraploid with ~hose presented by Hanna et al (1976), it is clear that the desynaptic tetraploid plant had a higher frequency of univalents. Mean chiasma frequency per cell in the different desynaptic diploids plants varied from 0.2 to 8.4 whereas in the desynaptic tetraploid a mean of 15.12 chiasmata per cell was observed which was nearly twice the maximum frequency observed for the desynaptic diploid plants, but compared to the normal diploids the mean chiasma frequency was only slightly higher (mean r frequency in normal 2n plants being about 12.82 per cell). The distribution of chromosomes at anaphase I was highly variable in both desynaptic diploids and desynaptic tetraploid. For this study, anaphase I cells without laggards were scored. The types of distribution of chromosomes in the desynaptie diploids ranged from 7-7 to 0-14 of which 8-6 distribution was more frequent (40.8 70). In the desynaptic tetraploid the distribution of chromosomes at anaphase I varied from 11-17 to 14-14 and the former type was more frequent (147o). The desynaptic tetraploid had more number of cells (59 70) showing laggards at anaphase I than the desynaptic diploid plants (18.9 70). The acetocarmine stained pollen grains ranged from 4 to 6870 in the different desynaptic diploid plants whereas it was about 1070 in the desynaptie tetraploid. A very low percentage of occasional seed set was observed in the different desynaptic diploid plants whereas the desynaptic tetraploid did not set seed either on selfing, on controlled pollination or in open pollinatiom. 4. Discussion The tetraploid used in the present study has occurred in the open pollinated progeny of a desynaptic diploid and it exhibited an amount of desynapsis. The origin of tetraploids from diploids is very rare but not unexpected. Harlon and De Wet (1975) in a recent review cited instances of such spontaneous occurrence of tetraploids. The occurrence of triploids from diploid stocks of pearl millet have been observed at frequent intervals (Krishnaswamy and Rangaswamy Ayyangar 1941; Pantulu 1967), but tetraploids have never been reported in untreated stocks of pearl millet. Triploids presumably arise through the functioning of an unreduced

Meiosis in pearl millet 19 Figures 1 to 4. Chromosome associations in desynaptic diploid and desynaptic tetraploid plants of Pennisetum typhoides. Diakinesis showing 1. 6 ring and 1 rod bivelents. 2. One rod bivalent and 12 univalents. 3. 5 quadrivalents and 4 bivalents. 4. 2 trivalents, 4 bivalents and 14 univalents.

Meiosis in pearl millet 21 egg and a normal haploid male gamete. Because of the infrequency with which triploids occur, the probability is very rare that a tetraploid would occur in a similar manner through the random mating of two exceptional gametes with diploid chromosome numbers, especially as diploid pollen cannot compete successfully with haploid pollen. However it is known that asynapsis and desynapsis cause meiotic failures of various kinds. In maize, elongate (el) is a gene on chromosome III that greatly increases the frequency of unreduced eggs (Rhoades 1956) and it has been used to develop an extensive series of tetraploid maize stocks for breeding purposes (Alexander 1957, 1958). Lesley and Lesley (1930) reported one tetraploid derived from a double trisomic stock of tomato. In pearl millet itself Hanna et al (1976) reported the occurrence of a tetraploid from a diploid. Their tetraploid was one of the twin seedlings, the other seedling being diploid. They expressed the view that the 4n plant might be the consequence of polyembryony and subsequent doubling of the chromosome number either during the process of fertilization (by means of unreduced gametes) or during the zygotic stage. They further suggested that the polyembryonie characteristic appears to be under genetic control. Pantulu and Narasimha Rao (1977) have reported, in pearl millet, the origin of a tetraploid from a triploid. As these tetraploids showed genetically controlled chromosome numerical mosaicism, they attributed the origin of tetraploid from triploid to this phenomenon. In the present study the origin of tetraploid from a diploid may be either due to the fusion of unreduced gametes or fusion between haploid gametes followed by doubling of chromosome number during the developmental stages of the zygote. Yet another possibility is spindle abnormalities during premeiotic mitotic divisions in a diploid plant leading to tetraploid PMCs. Ahloowalia (1969) studied desynapsis in diploid and tetraploid clones of ryegrass and assumed that the tetraploid showing desynaptic condition might be genotypieally Ds Ds ds ds and that there might be partial dominance of Ds over ds in the tetraploid. In diploid stocks of pearl millet the desynaptic character has been shown to be single gene controlled (Minocha et al 1975). As the present desynaptic tetraploid was obtained in open pollination from a desynaptic diploid, the genetic constitution of the female gamete with reference to the desynaptic character was ds ds. If the pollen happened to be from a normal diploid, the desynaptic tetraploid might be duplex or simplex as in ryegrass (Ahloowalia 1969). As the present desynaptic tetraploid was completely sterile it was not possible to know its genetic constitution. Morrison (1956) has suggested that tetraploids with asynaptic genotype may be expected to have reduced chiasma frequency accompanied by reduced multivalent frequency and increased bivalent formation, all of which together contribute to an improved fertility. But the present results show that the suggestions of Morrisoa do not hold good in this crop plant. This may be due to two reasons (1) formation of multivalents (though in low frequencies) even in the desynaptic tetraploid and (2) high frequency of univalents, both of which contribute to an unequal distribution of chromosomes at anaphase I leading to gametes with unbalanced chromosome numbers. Acknowledgements My sincere thanks are due to Dr J V Pantulu for his encouragement and valuable

22 M V Subba Rao guidance and to CSIR, New Delhi for the award of a fellowship during the tenure of this work. References Ahloowalia B S 1969 Meiosis in diploid and tetraploid clones of rye grass; Genetica, 40 379-392 Alexander D E 1957 The genetic induction of autotetraploidy: A proposal for its use in corn breeding; Agron. 3". 49 40-43 Alexander D E 1958 Genetic induction of autotetraploidy in maize; Proc. loth Int. Congr. Genet. Montreal: 3 (Abst). Darlington C D 1937 Recent Advances in Cytology, (London: J & A Churchill) Dhesi J S, Jagtar S, Gill B S and Sharma H L 1973 Cytological studies of desynaptic stock in pearl millet ( Pennisetum typhoides) ; Cytologia 38 311-316 Hanna W W, Paoell J B and Burton G W 1976 Relationship to polyembryony, frequency, morphology, reproductive behaviour and cytology of autotetraploid in Pennisetum americanum; Can. J. Genet. Cyto118 529-545 Harlan R, Jack and De Wet J M J 1975 On O Winge and A prayer: The origin of polyploidy; Bot. Rev. 41 361-391 Jauhar P P 1969 Partial desynapsis in Pearl millet, Pennisetum typhoides; Naturwissenschaften 56 571-572 Krishnaswamy N and Rangaswamy Ayyangar G N 1941 An autotriploid in Pearl millet (P. typhoides); Proc. Indian Acad. Sci. BI3 9-23 Krishnaswamy N, Raman V S and Madhavan Nair P 1949 Abnormal meiosis in Pennisetum typhoides I. Desynapsis; Proc. Indian Acad. Sci. 30 195-206 l.,e, sley M M and Lesley J W 1930 The mode of origin and chromosome behaviour in pollen mother cells of a tetraploid seedling tomato, J. Genet. 22 419-425 Morrison J W 1956 Chromosome behaviour and fertility of Tetra Petkus rye; Can. J. Agric. ScL 36 157-165 Minoeha J L, Dhesi J S and Sidhu J S 1975 Inheritance of desynapsis in Pearl millet; Indian J. Genet. Plant Breed 35 470-471 Pantulu J V 1967 Chromosomal alterations in Pearl millet induced by gamma-rays; Nature 213 101-102 Pantulu J V and Narasimha Ran G J 1977 Genetically controlled chromosome numerical mosaicism in Pearl millet; Proc. Indian Acad. Sci. B86 15-17 Patil B P and Vohra S K 1962 Desynapsis in Pennisetum typhoides; Curr. Sic. 32 345-346 Khoades M M 1956 Genetic control of chromosome behaviour; Maize Genet. Coop. Newslett. 30 38-42 Singh R B, Singh B D, Lakshmivijay and Singh R M 1977 Meiotic behaviour of spontaneous and mutagen induced partial desynaptic plants in Pearl millet; Cytologia 42 41-47