CYTOTAXONOMIC STUDIES ON THE GENUS BOONACRIS: I. The "eastern" taxa and a comparison with the related genera Dendrotettix and Appalachia (Orthoptera: Catantopidae: Podismini)

Abstract

Thirteen populations of four distinct taxa in the wingless grasshopper genus Boonacris with an eastern distribution in Canada and the United States had a chromosome complement of 2n ♀; ♂ = 20A + XO;XX. A detailed analysis of chromosome lengths and of the heterochromatin content and distribution in the karyotypes of the populations sampled revealed extreme endophenotypic stability and conservatism. Two populations were found to contain heterochromatically variant karyomorphs, one being polymorphic with respect to a complex pattern of autosome and sex-chromosome heterochromatin expression, the other for the occurrence of a large heterochromatic supernumerary chromosome. The cytogenetic system of Boonacris species was compared with those of the related, largely sympatric species Appalachia arcana and Dendrotettix quercus. The last two species have 2n ♀; ♂ = 22A + XO;XX but their complements show structural differences. The available evidence from a number of podismine species suggests that the forms with the lower chromosome number (i.e. 21 ♀; 22 ♂) may have originated from a common 'proto-Podisma' ancestor with 23 chromosomes (♀) via evolutionary loss or elimination of a small, unstable megameric pair. This dichotomy of major phyletic lineages and other cytogenetic evidence is discussed in relation to a phenetic classification of the tribe.