Chromosome rearrangements are not accompanied by expected genome size change in the tree weta Hemideina thoracica (Orthoptera, Anostostomatidae)

Abstract

A comparison of hybrid zones between chromosome races of the tree weta Hemideina thoracica (White) found that the chromosomal rearrangements that involved the greatest proportion of the genome apparently had the least effect on fitness of hybrids. In order to further explore the nature of chromosomal rearrangements that differentiate weta races, relative DNA content was compared between 2 races of H. crassidens (Blanchard) (known to differ by 2 Robertsonian (Rb) translocations) and 4 races of H. thoracica, using flow cytometry. Where Rb translocation was the inferred process (H. crassidens 15 vs 19; H. thoracica 17 vs 15′) no change in relative DNA content was detected, as expected. The 19-karyotype of H. thoracica was predicted to have more DNA due to a duplication/loss compared to the 17-karyotype of H. thoracica, but here too, no change in relative DNA content was detected. In contrast, significantly more DNA was found where less was predicted; the 13-karyotype of H. thoracica was thought to have less DNA than the 15′-karyotype, due to a duplication/loss, but in fact this race has significantly more DNA than any of the 3 other races of H. thoracica examined. Due to the differential staining of A-T bases by the stain used in flow cytometry, 4′-6′-diamidino-2-phenylindole (DAPI), it is possible the increased DNA estimate for the 13-karyotye is due to a significant shift in A-T base bias throughout its genome. However, it is more likely that the absence of the very small acrocentric autosomes that characterize many of the karyotypes of H. thoracica, result from translocations to larger autosomes than from a change in total DNA content and a simultaneous change in base ratio.