Euphytica 24 (1975) 149-157

NOTES ON THE CYTOGENETIC STRUCTURE OF THE CULTIVATED OAT AVENA SATIVA

(2n = 6x = 42)

HUGH THOMAS and I. M. BHATTI’

Welsh Plant Breeding Station, Aberystwyth, Wales, UK

Received 22 March 1974

SUMMARY

The cytogenetic structure of the cultivated oat has been studied using aneuploids. The chromosome pairing observed in pentaploid hybrids between the tetraploid species A. magna and telocentric lines of A. saliva show that the occurrence of multivalents in the hybrid are in part due to chromosomal differentia- tion.

Nullisomic tetrasomic compensation studies indicate homoelogous relationships of the constituent genomes of A. sativa. This is also apparent in attempts to substitute different chromosomes of A. saliva with a chromosome of the tetraploid species A. barbata.

The meiotic behaviour of a haploid plant of A. sativa shows that the diploid-like pairing observed in the hexaploids is under genetic control.

INTRODUCTION

The cultivated oat Avena sativu is an allopolyploid combining three related genomes designated A, C and D (RAJHATHY & MORRISON, 1960). The wild hexaploid species have the same basic chromosome structure as A. sutiva although some minor struc- tural differences are apparent from the meiotic behaviour of hybrids between hexa- ploid species (LADIZINSKY, 1970). However, they do not form distinct isolation bar- riers between the hexaploid species as at the diploid level (RAJHATHY & THOMAS, 1967). Chromosomal differentiation is a common feature at all levels of ploidy in the genus Avena and this makes it difficult to trace the putative diploid and tetraploid progenitors of the hexaploid species by studying chromosome pairing in interspecific hybrids. The polyploid structure of A. sutivu allows the species to tolerate chromoso- me loss and duplication and makes possible the isolation of aneuploid lines. (McGIN- NIS, 1966; HACKER & RILEY, 1966). Aneuploid studies in Triricum uestivum have resulted in an understanding of the basic genetic structure and the control of the reproductive processes of this hexaploid species (SEARS, 1954; RILEY, 1960). This report presents data from cytogenetic studies involving aneuploid lines which reveal the cytogenetic structure of the cultivated oat.

MATERIALS AND METHODS

A number of monosomic lines derived from different cultivars were used in these studies. The tetrasomic line used in the nullisomic tetrasomic compensation studies was derived from the cultivar Garry and was made available to us by Dr. R. C. MC

i Present address: Rice Research Institute, Dorki, Pakistan.

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H.THOMAS AND I.M.BHATTI

GINNIS, Department of Plant Science, University of Manitoba. The barbata addition line was isolated at the Welsh Plant Breeding Station from the progenies of a plant derived from backcrossing twice the A. barbata x A. sativa amphiploid to A. sativa. The polyhaploid occurred spontaneously in the progeny of a mono-trisomic geno- type and was nullisomic for the shortest chromosome of the complement and disomic for another.

For cytological studies immature panicles were fixed in Carnoy (6 : 3 : 1) solution. Anthers were stained in alcoholic hydrochloric acetocarmine stain (SNOW, 1963) and squashed in 45% acetic acid.

RESULTS AND DISCUSSION

Tetraploid progenitor. The hexaploid species of Avena probably originated from a natural hybrid between a diploid and tetraploid species with the subsequent spon- taneous doubling of the chromosome number or the fusion of unreduced gametes in the first place. None of the pentaploid hybrids involving tetraploid species of Avena and A. sativa have the regular chromosome pairing of 14 bivalents and 7 univalents as in the corresponding pentaploid hybrids in Triticum.

Two groups of tetraploids have been described, namely, the barbata complex and the magnalmurphyi complex. Pentaploid hybrids between A. barbata and A. sativa had a mean of 18.62~ 5.8511, 1.32111 and 0.171~ at metaphse ~“(THOMAS & JONES,

1964), and a low chiasma per paired chromosome. However, the pentaploid hybrids A. magna x A. sativa showed a higher degree of chromosome pairing and a higher value for chiasma per paired chromosome (MURPHY et al., 1968; RAJHATHY &

SADASIVAIAH, 1969). LADIZINSKY & ZOHARY (1970) proposed that A. magna probably represented the tetraploid background on which the evolution of the hexaploids is based. Although there is a higher degree of chromosome pairing in this hybrid compared with the A. barbata x A. sativa pentaploids, it deviated significantly from the 14rr + 71 expected if the two genomes were completely homologous. The maxi- mum chromosome pairing observed in A. magna x hexaploid species indicates that the two genomes of magna are related to two of the hexaploid species. Deviation from the expected 14u+ 71 is primarily due to the formation of multivalents in the pentaploid hybrids (Table I). Multivalents could be a reflection of translocations or homoeologous pairing.

A. magna was crossed with a number of telocentric lines of A. sativa and two parti- cular combinations were of interest in that they showed that multivalent formation in the pentaploids was probably due to structural differences. In these two lines, mono-9

Table 1. Chromosome pairing in pentaploid (interspecific) hybrids.

Cross Frequency of Source

I II III IV

A. barbata x A. saliva 20.25 4.69 1.42 0.23 THOMAS&JONES, 1964 A. magna x A. sativa 10.60 8.10 1.87 0.59 SADANAGA etal., 1968 A. magna x A. sterilis 8.13 1.77 2.33 1.11 RAJHATHY& SADASIVAIAH, 1969

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CYTOGENETIC STRUCTURE OF OATS

Table 2. Frequency of heteromorphic configurations in FL hybrids involving telocentric lines of mono-9 and mono-13 and A. magna.

Percent of pmcs in which telocentric was associated as

Number of cells

II III IV

mono-9 x A. magna mono-13 x A. magna

51.0 2.0 0.0 100 32.0 0.0 54.0 50

Fig. 1. a and b: Meiosis in a haploid plant of A. sativa, 1 rod and 1 ring bivalent in la, and one ring bivalent in 1 b. c and d: Meiosis in A. sariva (telocentric lines) x A. magna. In lc the telocentric is paired in a quadrivalent, and in Id in a bivalent (marked with arrow).

and mono-13, the telocentric chromosome paired with the magna chromosome (Table 2). The telocentric of mono-9 was paired as a heteromorphic bivalent (Fig. Id) in 57% of the pollen mother cells (pmcs) examined, However, the mono-13 telocentric was involved in a heteromorphic quadrivalent (Fig. Ic) in 54% of the pmcs scored and as a heteromorphic bivalent in 32% of the cells. The consistently high frequency with which the mono- 13 telocentric participated in the formation of a quadrivalent indicates that this particular sativa chromosome was involved in a

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H. THOMAS AND 1. M. BHATTI

translocation in one of the species. On the other hand the mono-9 telocentric was not involved in a translocation since it consistently formed a heteromorphic bivalent. As they involved the same genotypes and differed only in the aneuploid condition of the A. sativa, both hybrid combinations gave similar mean chromosome pairing values. If the formation of multivalents was due to homoeologous pairing the consis- tent participation of mono- 13 in a quadrivalent could only be explained in terms of larger differences in differential affinity between homeologues than that reported to occur in Triticum aestivum by RILEY & CHAPMAN (1966).

Major structural differences between corresponding chromosomes in A. magna and A. sativa would account for the deviation from the expected 1411t 71 based on the common ancestry of the magna genomes and two of the sativa genomes. It is impossible to draw any conclusions as to whether such structural divergence occurr- ed before or after the tetraploid species was involved in the evolution of the hexaploid species. A. magna might not have been directly involved in the evolution of the hexa- ploid species, but formed part of a complex which did contribute the actual tetraploid progenitor. The demonstration that the occurrence of multivalents in pentaploid hybrids was the result of structural differentiation supports LADIZINSKY & ZOHARY’S

(1970)postulation that A. magna belongs to the tetraploid base involved in the evolu- tion of the hexaploid species.

Evidence for homoeology. The hexaploid species of Avena are assumed to have origin- ated by the combination of three diploid genomes and their integration into a repro- ductively stable form. Since the diploid progenitors have not been precisely identified it is impossible to assess the degree of homology that exists between the diploid species involved in the evolution of the hexaploid species.

The ability of A. sativa to tolerate chromosome loss and duplication in a com- parable manner to wheat (MCGINNIS, 1966; HACKER & RILEY, 1966) shows that genetic duplication is also inherent in the cultivated oat.Studies have been initiated to classify the chromosomes of A. sativa into homoeologous groups following the procedures described by SEARS (1966). The complete series of monosomic lines is not yet available but the lines already identified were crossed with the tetrasomic line isolated by Dr R. C. MCGINNIS from the variety Garry. The resulting Fi hybrids, which were monosomic for one chromosome and trisomic for another, were back- crossed to the tetrasomic line to produce the mono-tetrasomic progeny. Nulli- tetrasomic plants were isolated from the selfed progenies of the mono-tetrasomic. Nulli-tetrasomic combinations involving different monosomic lines were compared morphologically with the corresponding nullisomic plants to detect nullisomic tetrasomic compensation. In two lines the nulli-tetrasomic progeny were as vigorous as euploid plants (Fig. 2b), although the corresponding nullisomics were extremely weak. In all other combinations the nulli-tetrasomic plants showed the deleterious effects of the nullisomics (Fig. 2a). In the first group the extra dose of the chromosome in the tetrasome compensates for the deletion of the other pair of chromosomes; this is not the case in the second group. The ability of the extra dose of chromosomes to compensate effectively for the loss of another pair is specific in that only two lines were found in which compensation was observed. Both nullisomics involved in the two compensating lines were sterile, and fertility was partially restored in the nulli-

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Fig. 2. Mature plants of the nullisomics, nulli-tetrasomic and monotetrasomics in compensating com- binations (2b) and noncompensating (2a).

tetrasomic genotypes. The ability of the tretasome to compensate for the deleterious effects shown by the nullisomics is a manifestation of the genetic equivalence of the chromosomes involved in the monosomic and tetrasomic lines. Although the data available relate to one set of crosses using only one tetrasomic line and the partial series of monosomics, they indicate a homoeologous relationship between the sativa consistuent genomes as reported in T. aestivum by SEARS (1966). Compensating ability is associated with the common ancestry of the constituent genomes although divergence in chromosome structure between the constituent genomes has occurred. Nevertheless, the genetic equivalence shown by compensating studies demonstrates that corresponding chromosomes in the three genomes have retained their basic genetic functions. These nullisomic tetrasomic studies provide the first unequivocal demonstration of homoeologous relationships in A. sativa.

Further evidence of homeology is apparent from attempts to substitute sativa chromosomes with chromosomes of related species. A disomic chromosome addi- tion line, i.e. the complement of the cultivar Manod plus a pair of barbata chromoso- mes was crossed with a range of monosomic lines of A. sativa. The gene for mildew resistance was located on the barbata chromosome. The subsequent hybrids which were monosomic for the barbata and sativa chromosomes were selfed, and mildew resistant genotypes in which a pair of sativa chromosomes had been replaced by the

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pair of burbata chromosomes were isolated from the progeny of the double monoso- mic. Several chromosome substitution lines involving a number of monosomic lines were isolated, but only three lines proved as vigorous as the euploid plants. The other substitution lines were as weak as the corresponding nullisomics. The barbata chro- mosome can only effectively compensate for the sutiva chromosome of the same homoeologous group.

Compensation was also apparent at the gametic level in that pollen which was nullisomic for the sutiva chromosome but included the barb&a chromosome could successfully compete with euploid pollen in the double monosomic plants. The high competitive ability of the mullisomic + 1 barb&a pollen resulted in a higher propor- tion of mildew resistant plants in the progeny of the double monosomic genotypes in the compensating than in the noncompensating combinations (Table 3).

Table 3. Proportion of resistant plants in the progeny of double monosomic plants (2011 + 1 I + 1 I).

Proportion of F plants resistant

compensating 0.324 non-compensating 0.115

A. barbata did not participate in the evolution of the hexaploid species and is genetically isolated from the latter in nature. Nevertheless, the barbatu chromosome involved in these studies had retained its basic genetic function and has been success- fully used in substitution iines involving genetically equivalent chromosomes in sativa. Althoug the whole series of monosomic lines is not available it is reasonable to conclude from the data that the substituting ability of the barbata chromosome is specific as defined by RILEY et al. (1966).

The control of chromosome pairing. Bivalent pairing and disomic segregation occur consistently in the hexaploids and the genetic duplication clearly demonstrated in nullisomic tetrasomic compensation is not expressed cytologically in the meiotic behaviour of A. sativa. Chromosome pairing is confined to true homologues, and homoeologous pairing is extremely rare. Although it is possible to explain bivalent pairing satisfactorily in terms of differential affinity in the hexaploids, the low frequency of chromosome pairing in a haploid isolated from a mono-trisomic plant (Fig. 1 a and b) does indicate that diploid-like pairing in the hexaploid is not entirely dependent on differential affinity. This particular haploid has one pair of chromoso- mes in the disomic state and is nullisomic for the shortest chromosome of the com- plement. The pair of chromosomes consistently form a ring bivalent. With the excep- tion of this ring bivalent, a rod bivalent was formed in only 5 pmcs out of the 93 analysed. Associations higher than bivalents were not recorded in a single pmt. NISHIYXMA & TABATA (1963) reported similar chromosome pairing in the haploid derived from the cultivar Kenata. The amount of chromosome pairing observed in the sativa haploid is lower than that reported in wheat polyhaploids (RILEY & CHAP- MAN, 1957). The low frequency of homoeologous chromosome pairing in polyha-

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ploids, when the homologues have been removed, shows that bivalent pairing in the polyploid species is genetically controlled.

A nullihaploid plant described by GAUTHIER & MCGINNIS (1968) had more chro- mosome pairing than the euhaploid. They postulated that the increased chromosome pairing was due to the absence of the chromosome which carries the gene or genes controlling bivalent pairing in the nullihaploid. This would be similar to the degree of chromosome pairing found in wheat haploids nullisomic for chromosome 5B (RILEY & CHAPMAN, 1958). The identification of the chromosome concerned and the precise description of the system involved in the control of chromosome pairing in A. sativa has not yet been accomplished.

In our study of the meiotic behaviour of the haploid the frequency of associations of univalents in pairs or groups was recorded. Side to side (s-s) secondary associa- tions of the univalents in the A. sativa haploid were less frequent than end to end (e-e) associations (Table 4). PERSON (1955), RILEY & CHAPMAN (1957) and UMBER & RILEY (1963) have shown that the frequency of s-s associations of univalents in polyhaploids was inversely correlated with the associations dependent on chiasmata.

Table 4. The frequency of different secondary associations of univalents in a polyhaploid of A. sativa.

Configuration

side to side (s-s) end to end (e-e)

Proportion of pmcs

0.41 1.65

Such a relationship shows that the chromosome involved in s-s secondary associa- tions are segmentally homologous or homoeologous. In this context it is of interest to observe that the frequency of s-s associations of univalents in the A. sativa polyha- ploid was lower than the corresponding polyhaploid in wheat, a mean of 0.41 per pmc in A. sativa compared with 1.32 and 1.73 in two haploids of T. aestivum (RILEY &

CHAPMAN, 1957). PERSON (1955) did not find a corresponding relationship between frequency of e-e associations and bivalents in haploid T. aestivum and concluded that the occurrence of e-e associations were not dependent on residual homology. In the A. sativu haploid the frequency of e-e associations was higher than s-s associations. If the incidence of s-s associations is dependent on initial prophase pairing and the subsequent failure of chiasma formation, the lower frequency of s-s associations in A. sativa would indicate greater structural divergence of the constituent genomes or a stronger inhibition of homoeologous pairing in A. sativa than in T. aestivum. Less chromosome pairing in the A. sativa haploid compared with T. aestivum would also be a reflection of the more diploidised structure of the hexaploid Avena.

The genetic control of bivalent pairing in A. sativa can be inhibited by a genotype of the diploid species A. Zongiglumis (Cw 57). RAJHATHY & THOMAS (1972) reported that the F1 hybrid between A. sativa and A. longiglumis (Cw 57) had more chromoso- me pairing than hybrids involving the latter with another genotype of A. longiglumis (Cc 485 1). Further hybrids between the intraspecific Fi hybrid between Cc 485 1 and Cw 57 and A. sativa segregated into either low or high pairing hybrids. The ability

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H. THOMAS AND I. M. BHATTI

of Cw 57 to suppress the mechanism promoting bivalent pairing in A. sativa is con- trolled by a single gene.

CONCLUSION

The use of aneuploid stocks of A. sativa to study the cytogenetic structure of the hexaploid species should prove to be as effective as their use in wheat cytogenetics. From the limited data already available from such studies it is possible to conclude that the three constituent genomes of the cultivated oat are genetically close. Regulari- ty of meiosis is not completely dependent on the allopolyploid origin of A. sativa but is under genetic control, which enhances differential affinity. By confining chromo- some pairing to homologues the system ensures the regular disjunction of chromoso- mes and disomic inheritance, but at the same time maintains considerable genetic duplications.

Understanding the cytogenetic structure of the cultivated oat will facilitate the development of techniques for the introduction of desirable alien variation into the cultivated species. The genetic control of chromosome pairing in the hexaploid species limits the chances of the recombination of specific characters in interspecific hybrids between the cultivated forms and the wild species of lower ploidy. Conventional breeding techniques, therefore, cannot be used to transfer desirable genes from related diploid and tetraploid species into the cultivated oat (THOMAS & THOMAS, 1972). It is imperative to understand the control of the reproductive processes of the polyploid in order to exploit the desirable variation found in the wild species of Avena. Studies which are being undertaken involving aneuploid stocks of A. sativa should indicate further the cytogenetic structure and the control of chromosome pairing in this im- portant crop species.

ACKNOWLEDGEMENTS

We wish to thank Professor P. T. Thomas, CBE., for his advice and encouragement throughout the study, and Dr D. J. Griffiths for his helpful discussion. I.M.B. was supported by the Colombo Fellowship Plan.

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