Amount of Heterochromatin as a Specific Character

Abstract

ONE general fact which emerges from, recent work in animal cytology is that the extent of the heterochromatic ('inert') segments of the chromosomes often varies very greatly from one species to another, even within the same genus. Thus, among the grasshoppers, Stauroderus scalaris possesses very large proximal heterochromatic regions in all the chromosomes, while in other species of the genus these regions are very much shorter and less conspicuous¹. The same difference has long been known to exist between species of Drosophila, where D. virilis has the proximal half of each autosome heterochromatic, while in some other species such as busckii there is far less heterochromatin. These species belong to different subgenera ; but it has recently been shown by Wharton² that D. melanopalpa has considerably more heterochromatin than the very closely related repleta. In the Pentatomids, Schrader³ has shown that Edessa irrorata has prominent heterochromatic blocks at the ends of most or all of its chromosomes, which are not obvious in several other members of the same genus. An identical situation occurs in the Corixidæ, where Slack⁴ has shown that Corixa punctata is a species with unusually extensive heterochromatin.