Cytogenetics of the grasshopper Moraba scurra. 1. Meiosis of interracial and Interpopulation hybrids.

Abstract

Numerous F1 hybrids between the races of Moraba scurra Rehn with 2nB = 15 and 2nB = 17 were reared in the laboratory and in artificial colonies established in nature. Most F1 males have almost completely regular meiosis, their chiasma frequency being essentially normal. These individuals show a trivalent, composed of an "A" and a "B" chromosome of the 17-chromosome race paired with the corresponding limbs of an "AB" chromosome derived from the 15-chromosome race. Disjunction of the trivalent is usually extremely regular. Occasionally either the A or the B chromosome fails to pair with the AB. In hybrid individuals which happen to have their CD chromosome pair heterozygous for the Blundell rearrangement (i.e. Standard/Blundell) the proximal end of the A chromosome is occasionally paired with the Blundell-carrying CD chromosome. This suggests that the evolutionary "dissociation" of the AB into separate A and B chromosomes, which is present in the 17-chromosome race, was not a simple fragmentation but a translocation involving a Blundell chromosome, and that the A chromosome received its centromere from the Blundell element. Various types of abnormal synapsis occur in hybrids between individuals from populations situated several hundred miles apart, even when both belong to the 15-chromosome race. Chiasma-formation in CD bivalents homozygous and heterozygous for the three known sequences of the CD chromosome (Standard, Blundell, Molonglo) suggests that Blundell and Molonglo are both related to Standard as pericentric inversions. The bearing of these data on the history of the species is discussed.