CHIASMA FREQUENCY IN SPECIES AND SPECIES HYBRIDS OF AVENA

Abstract

(i) Chiasma frequency at metaphase has been studied in the following species and species hybrids: A. strigosa Schreb.; A. brevis Roth; A. Wiestii Steud.; A. barbata Pott; A. abyssinica Hochst. (2 strains); A. sterilis L. (2 strains); A. sativa L. var. Radnorshire Sprig; a triploid hybrid, A. barbata × A. strigosa; a pentaploid hybrid, A. abyssinica "naine" × A. sterilis maxima; and a hexaploid hybrid, A. sterilis (white) × A. sativa var. Radnorshire Sprig.(ii) The various factors that may affect chiasma frequency are discussed and it is concluded that there may be some parallelism between closeness of relationship and similarity in chiasma formation, as genetical control is one factor affecting the process. The results are examined in the light of this conclusion.(iii) The homogeneity of species in respect to proportion of bivalents with zero, one, two, three, or four chiasmata is tested by means of χ2. The test indicates no significant difference between the closely related diploid species, A. brevis and A. strigosa, but both differ significantly from the less closely related A. Wiestii. The differences between the tetraploid species are of questionable significance, those between the hexaploids, insignificant. It is suggested that the lack of significant differences among these last-mentioned forms may be due to a multiplication, in the polyploid, of genes favoring high chiasma frequency. In this way a maximum effect might be brought about, and inter-specific differences with respect to this character would consequently be lacking.(iv) High chiasma frequency in the hybrid might be regarded as strong evidence of homology, and consequently of relationship between parental forms. The triploid hybrid, A. barbata × A. strigosa shows a high chiasma frequency at metaphase, a condition consistent with the close relationship of the parental forms. The pentaploid hybrid, as might be expected, has a low chiasma frequency. The chiasma frequency of the hexaploid hybrid, A. sterilis × A. sativa var. Radnorshire Sprig does not differ significantly from that of its low-frequency parent, A. sterilis. Cytological observations in this hybrid, therefore, reveal only a slight degree of non-homology between parental chromosomes. It is noted that there are certain characters of A. sativa var. Radnorshire Sprig that suggest affinity with A. byzantina, a species commonly associated with A. sterilis in relationship schemes.