Weta Karyotypes: the Systematic Significance of Their Variation

Abstract

Karyotype variation is described within three species of weta: Hemideina thoracica, H. crassidens and Deinacrida connectens. The systematic and biological implications of this variation are examined by study of these species using allozyme electrophoresis and by a wider analysis of the karyotype evolution within Deinacridinae. Within H. thoracica nine karyotypes are described and the majority of populations have a single karyotype. Geographic distribution of the karyotypes suggests a single origin for each, with the exception of the 17-karyotype which has a disjunct distribution. Gene flow within chromosome races is no higher than it is between chromosome races. Phylogenetic analysis of H. thoracica populations does not show a strong concordance with the chromosome races. Gene flow is possible through the contact zone of the 15-karyotype with the 17-karyotype beside Lake Taupo, although the hybrid zone is narrow. Spontaneous addition of extra small acrocentric chromosomes occurred in F1 hybrids between the l9-karyotype and the 15-karyotype. Although some heterozygote disadvantage is hypothesed, little evidence of genetic integrity for the majority of the chromosome races within H. thoracica was found. Within the nonthern two-thirds of the range of H. crassidens, two karyotypes are described: the 15-karyogrpe is paraphyletic with respect to the 19-karyotype. Two Robertsonian translocations are the most likely explanation for the differentiation, and F1 hybrids are likely to be fertile. Elements that distinguish the three tree weta species of the North Island are compared to the characteristic of the chromosome races within these species. Seven karyotypes are described within D. connectens and four populations are apparently karyotypically polymorphic. The karyotype, morphological and alloryme variation reveal three different patterns of differentiation within this species. Morphological variation shows little geographic pattern, but the distribution of allozyme and karyotype forms suggest differentiation was initiated in a more continuous population or that occasional or continuous contact has allowed gene flow between populations. A comparison of the karyotypic variation within four weta species reveals variation in the rate of chromosome evolution in this lineage. Estimates of gene flow within the four species do not differ significantly for most comparisons but D. connectens may have a higher rate of gene flow than H. thoracica and H. crassidens. Factors, other than fixation rate, that might control rates of chromosome change, such as mutation rate and selection, are discussed. Phenetic and cladistic analysis of allozyme data for 17 weta species produced very similar trees. Five clades of closely related species pairs or trios are obtained. The genetic phylogenies agreed with current taxonomy and with much of the evolutionary hypothesis based on stridulatory structures. No association between speciation and chromosome variation could be shown. The diploid number 25 is suggested as ancestral to the Hemideina clade. The rapid rates of chromosome evolution in D. connectens and H. thoracica are apparently due to convergence.