Cytogenetics of the grasshopper Moraba scurra. 3. Distribution of the 15-17 -chromosome Races.

Abstract

The 15- and 17-chromosome races of the wingless grasshopper Moraba scurra Rehn occupy areas which meet along a line at least 150 miles long, approximately separating the Southern Tableland of New South Wales from the south-western slopes. Within each area the species is at present confined to small colonies which only occur where grazing by sheep has been light or absent; but the populations of both races must have been far larger and more continuous a century ago. In the area between Yass and Rugby numerous small colonies of the 15- and 17-chromosome races have been found, only a few miles apart, and in one instance separated by approximately 900 yd. In spite of this, only a single interracial hybrid (i.e. a 16-chromosome male) was encountered and no colony was found containing representatives of both races. It is concluded that selection against chromosome-number heterozygotes (i.e. "negative heterosis") must be relatively strong. The distribution of the species as a whole in this area a hundred years ago must have resembled a jigsaw puzzle with perhaps a third of the pieces missing. Across such a pattern a zone possibly only a few hundred yards wide (and at most one or two miles wide) would have extended, within which overlap and interracial hybridization could have occurred. The origin of the 17-chromosome race is discussed in the light of these findings and interpretations.