Male transmission of the translocated chromosome in a tertiary trisomic of rye: genetic variation and relation to the rate of development of aneuploid pollen grains.

Variation in male and female transmission of the translocated extra chromosome (5R(3R)) was studied in a tertiary trisomic of rye (Secale cereale L.). In two F5 lines derived from a single F4 line, female transmission was lower than in five others derived from another F4 line. This could be caused by genetic factors or by the strong inbreeding depression in these lines, leading to low viability of trisomic progeny. After selfing, male transmission was estimated as very low, but this was primarily based on the occurrence of tetrasomics that probably have a very poor viability. In testcrosses with disomic female parents, male transmission was much higher (up to 27%), without variation within F5 lines. One F5 line showed significantly higher male transmission than any of the seven tested, including a sister line from the same F4. This was consistent in the F6. Apparently high male transmission is genetically determined. There was a positive correlation with recombination of the marker ti (tigrina) on the extra chromosome and the normal 5R chromosomes. At the first meiotic metaphase, trivalents and quinquivalents were frequent in the trisomics. Assuming loss of univalents, 40% of the microspores should carry the translocated extra chromosome. In most lines, more than 40% were found at pollen mitosis. Observations on timing of pollen mitosis showed a delayed development in aneuploid spores, with clear differences between plants, but no correlation with male transmission. The cause of reduced male transmission and the expression of genetic variation therein can, therefore, not be found in meiotic behaviour or delayed microspore development. Pollen germination and tube growth may be more important.