The Relationship of Homologous Synapsis and Crossing over in a Maize Inversion

Frequency of homologous synapsis at pachytene for a relatively short heterozygous inversion was compared to the frequency of crossover occurrence within the inversion and to the frequency of the presence of a recombination nodule within the homologously synapsed inverted region. Crossover frequencies were estimated from bridge-fragment frequencies at anaphase I and anaphase II. Recombination nodules (RNs) were observed in electron micrographs. Results show very similar frequencies of homologous synapsis and the occurrence of reciprocal recombination within the inverted region, consistent with the interpretation that establishment of homologous synapsis in this case is related to at least commitment to the form of resolution of crossover intermediates which gives rise to reciprocal recombination, not conversion only, events. An RN was generally found at pachytene in homologously synapsed inverted regions.     

Crossing over and Diploid Egg Formation in the Elongate Mutant of Maize

Rhoades (1941) found recombination in the proximal regions of chromosome 5 to be higher in male than in female flowers. Two explanations were proposed to account for the lower female values, namely: (1) there is a basic difference in rates of crossing over in mega- and microsporocytes, or (2) selective orientation of the chromosome 5 bivalent on the meiotic spindle leads to the preferential segregation of noncrossover chromatids to the basal megaspore. These alternatives have been tested by carrying out a half-tetrad analysis of the diploid eggs produced by plants homozygous for the recessive elongate (el) allele. The A2-Bt crossover values determined from the diploid eggs of elongate plants were much lower than those calculated from haploid sperm of both El el and el el plants. Since male and female flowers should have similar cross-over values if the orientation hypothesis were correct, it was concluded that the amount of crossing over in the A2-Bt region of chromosome 5 is intrinsically higher in male than in female meiocytes. In the analysis of diploid eggs the use of the Bt locus, which marks the centric region of chromosome 5, provided information on the origin of diploid eggs. The genotypic constitution of 425 diploid eggs was ascertained. Of these, 20.4% were Bt bt. They could not be accounted for by failure of the second meiotic division or by replication during the interphase between the two meiotic divisions, but are expected if there is a single division with an equational separation of the centromere regions of chromosome 5. The Bt Bt and bt bt genotypes arise from a disjunctional separation. It is proposed that diploid eggs are produced by an abnormal meiosis in which there is one division with either disjunctional or equational separation. Disjunctional separation is more frequent but the ratio of the two types varies from ear to ear. Recombination in the A2-Bt-Pr region of chromosome 5 was found to be higher in the haploid gametes of elongate homozygotes than in El El and El el plants. On the other hand, crossing over was reduced in the Sh-Bz segment of chromosome 9 in elongate plants, but the adjacent Bz-Wx interval was unaffected.