Genetic analysis of hybrid sterility in crosses of the tsetse flies Glossina palpalis palpalis and Glossina palpalis gambiensis (Diptera: Glossinidae)

Reciprocal crosses of Glossina palpalis gambiensis Vanderplank and Glossina palpalis palpalis (Robineau-Desvoidy) were carried out using flies that had four marker genes on the X chromosome, two in linkage group II and one in linkage group III. The results of the reciprocal crosses conformed to Haldane's rule: F-1 males were sterile and most F-1 females were fertile. F-1 females mated to G. p. gambiensis were more likely to be fertilized than females that were mated to G. p. palpalis. In three of the four experiments, the fertility of backcross females was not significantly different from that of F-1 females, and there was little evidence that specific chromosomal combinations influenced the fertility of backcross females. Intrachromosomal recombination was lower in hybrid females than in G. p. palpalis. The major genetic factor associated with sterility among backcross males was the presence of sex chromosomes from two subspecies; a minor factor was the number of heterozygous autosomes, but interactions between sex chromosomes and autosomes from different taxa did not contribute to hybrid male sterility. Evidence is presented that a major factor causing hybrid male sterility lies between the loci ran (an eye color) and Est-t (testicular esterase) on the X chromosome. The use of differences between the fertility of males produced by backcrossing F-1 females to the two parental subspecies as indicators that other X chromosome loci have a role in hybrid sterility is discussed.