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

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Abstract
Thirteen populations of 6 taxa in the wingless grasshopper genus Boonacris [B. glacialis glacialis (Scudder), B. g. canadensis (E.M. Walker), B. g. amplicera (Caudell et Allard), B. alticola Rehn et Randell, B. variegata (Scudder) and B. polita (Scudder)] with an eastern distribution in Canada and the USA had a chromosome complement of 2n .male.;.female. = 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 contained 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 spp. was compared with those of the related, largely sympatric species Appalachia arcana Hubbel et Cantrall and Dendrotettix quercus (Packard). The last 2 spp. have 2n .male.;.female. = 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 .male.;22 .female.) may have originated from a common proto-Podisma ancestor with 23 chromosomes (.male.) 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.