Solving the mystery of heterochiasmy

Genetic variation within a population is the difference in DNA sequence between individuals. Genetic variation is created in germ cells undergoing a specialised type of cell division called meiosis. During meiosis, numerous programmed DNA double strand breaks (DSBs) occur across the genome. A repair mechanism, specific to meiosis, promotes the pairing of chromosomes and mediates the repair of DSBs using the other chromosome as a DNA template. During this process, a subset of recombination events form crossovers, which are reciprocal exchanges of genetic information between two chromosomes. Crossovers change the DNA sequence of the genome, thereby creating genetic variation in each new generation. This, in turn, provides a range of physiological differences between offspring, enabling certain individuals to adapt to environmental changes. Hence, meiotic recombination is a pivotal part of both natural evolution and breeding.

Heterochiasmy is defined as the difference in recombination rate between male and female meiocytes. This phenomenon has been described in a wide range of species across the animal and plant kingdoms for over a century. Heterochiasmy is thought to be linked to sexual selection, and studies have shown that it varies in magnitude and direction among taxa, but its origin during evolution cannot be explained. Cross-kingdom comparison of meiotic chromosomes led to the striking observation that crossover number co-varies with chromosome axis length between the sexes. For example, mice female meiocytes have more crossovers and longer chromosome axes than male meiocytes, whereas the plant Arabidopsis meiocytes have fewer crossovers and shorter chromosome axis length in females than males. The current thinking in the field is that the chromosome axis could act on the establishment of heterochiasmy. However, co-variation of chromosome axis length with crossover rate does not demonstrate a causative role and it is possible that these two parameters are determined by a third (or more) factor(s) yet to be discovered. Indeed, recent studies showed presence of sex-biased recombination rate without co-variation with chromosome axis length in some bird and mouse species, thus challenging the paradigm of heterochiasmy. Other studies have shown that recombination rate can be influenced by varying amounts of meiotic proteins and DNA methylation. Therefore, alternative models should be considered to explain the cause of heterochiasmy.