Evolutionary history of Drosophila buzzatii. XV. Meiosis of inversion heterozygotes in backcross hybrids between D. buzzatii and its sibling D. koepferae

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Abstract
Interspecific F1 hybrid females between Drosophila koepferae and D. buzzatii are fertile, but hybrid males are sterile. By successive backcrossing of hybrid females to D. buzzatii males, it was possible to reduce progressively the genomic contribution of D. koepferae to the hybrid karyotype. Finally, only selected chromosome sections containing inversions restricted to D. koepferae were individually left in the otherwise D. buzzatii karyotype, namely: 2 C2b-F4a (j9m9n9), 3 Ca5-G1b (k2), 4 E2a-G2f (m), and 5 Cd5-F2h (.hivin.gw). Any of these sections produces hybrid male sterility in heterozygous condition. The present paper deals with the influence of these chromosome sections on meiosis. The observed abnormalities varied according to the introgressed segment borne by the hybrid males. The section from chromosome 2 of D. koepferae representing about 6.7% of the total polytene karyotype, had the most drastic effect, giving rise to the same phenotype observed in the F1 hybrids, which was characterized by the complete lack of meiotic stages. The disruption produced by the chromosome section containing inversion 3 k2 (4.9% of the karyotype) was only a little more severe than that containing 5 .hivin.gw (4.2% of the karyotype), and both may be distinguished only quantitatively. Anaphase I exhibited precocious separation of sister chromatids and aberrant segregations; metaphase II was also always abnormal, with the chromosomes separated into their constituent chromatids and no apparent junction between sister centromeres; anaphase II rarely took place, and then it was always highly abnormal, with aberrant segregations and frequent tripolar or tetrapolar spindles; finally, cytokinesis also failed quite often, giving rise to multinuclear spermatids. The introgression of the chromosome segment containing inversion 4 m (3.2% of the karyotype) produced the least deviation from normal meiosis, which consisted simply of a relative increase in the number of metaphase II among the cells undergoing meiosis. These results show a positive correlation between the size of the introgressed segments and the importance of the disturbances they bring about. A similarity with some described meiotic-sterile mutations is suggested.