Male and female meiosis evince differential patterns in chiasma formation: a case study of ornamental plant, Delphinium ajacis L.

Chromosomal behaviour during megasporogenesis and microsporogenesis has been studied in ornamental Delphinium ajacis L. Meiosis in female sex cell initiates later than male. The floral buds which carry egg mother cell (EMC) at diplotene stage has pollen mother cells (PMCs) at tetrad stage of meiosis suggesting protandry. Although the 16 chromosomes formed regular eight bivalents in both the sex cells, they differed in overall chiasma frequency which was 32.95% higher in EMCs and found to be 18.52 ± 2.12 per cell. In PMCs, the average chiasma frequency recorded was 13.93 ± 1.40 per cell. Interestingly, this variation in chiasma frequency was largely confined to the two large bivalents which shared 42.61% chiasma per EMC. The use of Q–Q plot, Box plot and Whisker plot showed departure in the chiasma frequency distributions in EMCs and PMCs from the normal distribution pattern. The difference in chiasma frequency in the two sex cells was significant at all levels as indicated by the low P values of 3.094 9 10-11 obtained from nonparametric test, i.e. Wilcoxon rank-sum test. It is suggested that the two different mechanisms of recombination are operational in the two sex cells, and the sex differences of chiasma frequency could have arisen due to differential epigenetic modifications of the chromatin which pattern the double-strand breaks, and the position and frequency of crossing over visible as chiasmata.

Drosophila ananassae: a species characterized by spontaneous male recombination in appreciable frequency

Mutation and recombination are primarily responsible for generating the genetic variability in natural populations of microorganisms, plant and animal species including humans. Upon such genetic variations, elemental forces of evolution such as natural selection, random genetic drift and migration operate to bring about micro-evolutionary changes. Recombination or crossing-over produces new combinations of genes due to interchange of corresponding segments between nonsister chromatids of homologous chromosomes, thus, it is an important evolutionary factor. Since the time of T. H. Morgan, Drosophila has been subjected to extensive investigations on crossing over while employing a number of markers, which were used for gene mapping. Interestingly, recombination occurs in females of D. melanogaster but not in males. Later on, male crossing over was investigated in various species and its occurrence was reported in D. melanogaster, D. ananassae, D. simulans, D. willistoni, D. littoralis and D. bipectinata. Recombination occurs at very low rate in all these species except for D. ananassae, which shows spontaneous male crossing over in appreciable frequency, which is meiotic in origin. This unusual phenomenon in D. ananassae is influenced by various genetic factors as well as it shows strain variation as far as frequency of male recombination is concerned. Further, the presence of chiasmata during meiosis in males at a frequency capable of accounting for the observed recombination frequency extends evidence for meiotic origin of recombination in males of D. ananassae.