Relationship between pachytene synapsis, metaphase I associations, and transmission of 2B and 4B chromosomes in rye

2B rye plants selected for high (H) or low (L) B transmission rate were stu died at pachytene and metaphase I of meiosis to determine the relationship between synapsis, bivalents at metaphase I, and B transmission rate. The re sults show that the 2 B chromosomes (Bs) form bivalents at pachytene in bot h the H and L lines, whereas the frequency of bivalents at metaphase I is m uch higher in the H than in the L line. This demonstrates that B transmissi on is mainly related to the proper association of Bs at metaphase I, as wel l as that synapsis of the 2 Bs in the L line is normal, but the bivalent is not consolidated by a chiasma in most cases. Crosses were made between 2B plants of the H and L lines in all combinations (H + H, H + L, L + H, and L + L) to obtain 4B plants. Similarly, bivalent formation at pachytene and m etaphase I was studied. The results show that 4B plants of the H + H and L + L classes differ significantly at pachytene and metaphase I since the for mer forms more bivalents. The heterozygous 4 Bs of the H + L and L + H clas ses show intermediate values. The relation H + H > H + L > L + H > L + L wa s consistently found for the variables transmission rate, bivalents at pach ytene, bivalents at metaphase I, and B mean chiasma frequency. A maternal e ffect was also found. Our data suggest that there are two separate mechanis ms acting upon synapsis and chiasma formation in H and L B chromosomes: (i) there is variable efficiency of the control of synapsis at early stages of meiosis; and (ii) there is variable efficiency of the control of the numbe r of chiasmata.