Cytogenetics of the endemic New Zealand frog, Leiopelma hochstetteri: extraordinary supernumerary chromosome variation and a unique sex-chromosome system

Abstract

Cytogenetic data from 6 populations demonstrated unusual supernumerary chromosome variation in the primitive frog, Leiopelma hochstetteri. Frogs from the Coromandel Peninsula of the North Island of New Zealand averaged very high numbers of supernumerary chromosomes while individuals from other populations outside of the Coromandel region rarely had more than 1 distinctive supernumerary chromosome found only in females. The maximum number observed was 16 supernumeraries, present in 1 individual from Mt. Moehau. Supernumeraries showed meiotic instability as they failed to pair during prophase I in spermatocytes. In lampbrush preparations from oocytes, supernumeraries appeared as univalents or as highly unusual stellate aggregations consisting of up to 7 chromosomes joined at their telomeres. Lateral loops on lampbrush supernumeraries indicated transcriptional activity. Contrary to a previous hypothesis, high supernumerary chromosome numbers in L. hochstetteri were not correlated with meiotic abnormalities. Neither were supernumerary chromosomes correlated with variations in heterochromatin distribution in the regular chromosomes. Rather, heterochromatin distribution was shown to vary geographically between populations. Sex determination in L. hochstetteri was found to be through a supernumerary, univalent W chromosome. Females in all populations invariably had 1 distinctive supernumerary chromosome not present in males. This chromosome could be distinguished from other supernumerary chromosomes by distinctive C-banding patterns and larger size. The W chromosome has undergone more rapid evolutionary change than the autosomes. Both telocentric and metacentric iso-chromosome forms were found in most populations. Heterochromatin distribution on the W chromosome varied between populations, from very little heterochromatin restricted to the centromere in Coromandel populations to an almost completely heterochromatic W chromosome among frogs from the East Cape region. In lampbrush preparations, the W chromosome was morphologically distinct from other supernumeraries. Loss of a Z chromosome leading to a univalent sex-determining W chromosome is difficult to explain through prevailing theories of sex-chromosome differentiation. The 0W♀/00♂ sex-determination system of L. hochstetteri appears to be unique among animals.