Sexual dimorphism of body size and sternopleural bristle number: a comparison of geographic populations of an invasive cosmopolitan drosophilid

Abstract

Zaprionus indianus is a cosmopolitan drosophilid, of Afrotropical origin, which has recently colonized South America. The sexual dimorphism (SD) of body size is low, males being almost as big as females. We investigated 10 natural populations, 5 from America and 5 from Africa, using the isofemale line technique. Three traits were measured on each fly: wing and thorax length and sternopleural bristle number. Two indices of SD were compared, and found to be highly correlated (r > 0.99). For the sake of simplicity, only the female/male (F/M) ratio was further considered. A significant genetic variability of SD was found in all cases, although with a low heritability (intra-class correlation of 0.13), about half the value found for the traits themselves. For size SD, we did not find any variation among continents or any latitudinal trend, and average values were 1.02 for wing length and 1.01 for thorax length. Bristle number SD was much greater (1.07). Among mass laboratory strains, SD was genetically much more variable than in recently collected populations, a likely consequence of laboratory drift. Altogether, SD, although genetically variable and prone to laboratory drift, is independent of size variations and presumably submitted to a stabilizing selection in nature.

Divergent abdominal bristle patterns in two distantly related drosophilids: antero-posterior variations and sexual dimorphism in a modular trait

“…In Z. indianus female, the bristle numbers are on average quite similar to those found in D. melanogaster, in spite of its larger body size (Karan et al 2000;David et al 2006). A major difference is however observed on A7: on average 20.9 bristles in Z. indianus, but only 10.0 in D. melanogaster.…”

“…Sexual dimorphism for any trait can be described by considering either the female-male difference (F-M) or the F/ M ratio. It is currently considered that a ratio may be a better descriptor of dimorphism, since it eliminates the scaling effects due to size (Frankham 1968a, b;Reeve and Fairbairn 1996;David et al 2003David et al , 2006Huey et al 2006). Thus, for the sake of simplicity, we present here only the ratio.…”

Phenotypic plasticity of abdomen pigmentation in two geographic populations of Drosophila melanogaster: male–female comparison and sexual dimorphism

“…The overall value of the intraclass correlation (0.17) is significantly less than for the traits in each sex (0.22). Such a difference seems a general property of SD of various other traits, such as body size and bristle number (David et al , 2006b. The value significantly greater than zero seems however sufficient to permit an evolution by natural selection, when needed.…”

“…There are two major ways to describe SD, either among species or among full-sib lines: either the female-male difference in the mean values, or a ratio such as the female/male ratio (see David et al 2003David et al , 2006b. For a metrical trait such as thorax length, a ratio appears a more convenient estimate (Reeve and Fairbairn 1999;David et al 2003David et al , 2006bHuey et al 2006) since it is not sensitive to scaling effects. For pigmentation, the utilization of a ratio was not always convenient because, at extreme temperatures, the pigmentation might be sometimes completely black or completely light (scores of 10 or 0).…”

“…Up to know, there is only little information on the genetic variability of SD among populations. A recent investigation of body size and sternopleural bristle number in D. melanogaster and Zaprionus indianus (David et al 2006b) failed to evidence any variation among geographic populations and it was suggested that SD might be submitted, in nature, to some strong stabilizing selection. Our results on pigmentation are at odds with observations on body size and a major question remains: can we consider the difference in SD pigmentation as an adaptation that is a consequence of different selective pressures on the two sexes?…”