Spermatogenesis in natural and experimental hybrids between chromosomally differentiated taxa of Caledia captiva

Abstract

The “Moreton” and “Torresian” taxa of Caledia captiva are distinguished by multiple chromosomal differences involving a series of pericentric rearrangements together with differences in heterochromatin content. The role of these differences, in terms of their capacity to produce a postmating isolating mechanism between the two taxa was tested by an examination of male meiosis in F₁ hybrids and in field collected backcross derivatives. Multiply heterozygous F₁ hybrids, whether field collected or laboratory synthesised, displayed a low, but variable level of meiotic anomalies, principally in the form of univalent formation failure and the production of multiple associations as a result of intragenomic pairing. There was considerable heterogeneity between individuals and crosses in the frequency of such failure. None of the chromosomal differences between the taxa were implicated in the occurrence of univalent formation or the production of multiple association which implies that genotypic imbalance, rather than structural heterozygosity, is responsible for the observed anomalies of male meiosis in the hybrids. The chromosome differences between the taxa are therefore not considered to be involved in postmating isolation between them and hence cannot have played a direct role in their cladogenesis.