Mesosternal bristle number in a cosmopolitan drosophilid: an X-linked variable trait independent of sternopleural bristles.

Mesosternal (MS) bristles in Drosophila are a pair of machrochaetae found at the sternal end of the sternopleural (STP) microchaetae, and are thought to be invariable. In a closely related drosophilid genus, Zaprionus, their number is four and, in contrast to Drosophila, they show interspecific and intraspecific variability. The genetic basis of MS bristle number variability was studied in Z. indianus, the only cosmopolitan species of the genus. The trait responded rapidly to selection and two lines were obtained, one lacking any bristles (0-0) and the other bearing the normal phenotype (2-2). Other symmetrical phenotypes, (1-1) and (3-3), could also be selected for, but with lesser success. By contrast, STP bristle number did not vary significantly between the two lines (0-0) and (2-2), revealing its genetic independence from MS bristle number. Reciprocal crosses between these two lines showed that MS bristle number is mainly influenced by a major gene on the X chromosome (i.e. F(1) males always resembled their mothers) with codominant expression (i.e. heterozygous F(1) females harboured an average phenotype of 2 bristles). However, trait penetrance was incomplete and backcrosses revealed that this variability was partly due to genetic modifiers, most likely autosomal. The canalization of MS bristle number was investigated under different temperatures, and the increased appearance of abnormal phenotypes mainly occurred at extreme temperatures. There was a bias, however, towards bristle loss, as shown by a liability (developmental map) analysis. Finally, when ancestral and introduced populations were compared, the latter were far less stable, suggesting that genetic bottlenecks may perturb the MS bristle number canalization system. MS bristle number, thus, appears to be an excellent model for investigating developmental canalization at both the quantitative and the molecular level.

Evolutionary genetics of Zaprionus. II. Mitochondrial DNA and chromosomal variation of the invasive drosophilid Zaprionus indianus in Egypt.

Zaprionus indianus is an Afrotropical drosophilid species that has expanded its geographical range in the Palearctic region and the Americas during the second half of the last century. It has invaded Egypt within the past two decades from East Africa or Asia and became a dominant species in the drosophilid fauna therein, but the exact date of introduction and source of the propagule remain unknown. Here, we investigate the genetic structure of eight geographical populations within and around the Nile Delta using mitochondrial DNA sequences of the cox2 gene and chromosomal inversion polymorphism. A very low level of genetic variability was detected for both markers, mainly attributed to the introduction bottleneck. Nonetheless, both indicate a significant population structure, with a southeastern-northwestern cline. Demographic history analysis suggested northwestern populations to be younger (expanding in ca. 1992) than southeastern ones (expanding in ca. 1985). The In(II)A polymorphism was only observed in the northwestern population, but one-year interval analysis of the Alexandria population revealed the lack of seasonal fluctuation and a trend toward the loss of the polymorphism. Based on these data and faunistic records, we propose a multiple introduction scenario for Z. indianus in Egypt-according to which a first wave in the early 1980s from Sudan through normal northward range expansion or fruit trade, and a second wave in the early 1990s from Asia via fruit trade. We also suggest, from ecological observations, fruit trade data and known adaptive versatility of Z. indianus, date palm, the dominant fruit in Egypt and in the oases where Z. indianus predominates, to play a major role in the spread of the species in the Middle East.