Study of regional differences in GC content values in chromosomes of the guppy and related fish species

Genetic and physical mapping of the guppy (P. reticulata) have shown that recombination patterns differ greatly between males and females. Crossover events occur evenly across the chromosomes in females, but in male meiosis they are restricted to the tip furthest from the centromere of each chromosome, creating very high recombination rates per megabase, similar to the high rates in of pseudo-autosomal regions (PARs) of mammalian sex chromosomes. We here used the intronic GC content to indirectly infer the recombination patterns on guppy chromosomes. This is based on evidence that recombination is associated with GC-biased gene conversion, so that genome regions with high recombination rates should be detectable by high GC content. We used intron sequences and 3rd positions of codons, in order to make comparisons between sequences that are matched, as far as possible, with respect to selective constraints. Both these types of sites are likely to be under weak selection. Almost all guppy...