Abstract. We have investigated the effects of experimental manipulation of copulation duration on sperm displacement in Drosophila melanogaster. Both spermless and normal males were used as second (displacing) males in the experiments. Displacement induced in the absence of sperm, that is, by males that pass accessory gland fluid alone, was a relatively inefficient process and produced much lower levels of displacement than normal males. Therefore, the presence of second-male sperm is necessary (but unlikely sufficient) for the high levels of displacement commonly observed in D. melanogaster. Furthermore, when second matings were interrupted at various times after the initiation of copulation, the distribution of displacement was strongly bimodal. We conclude that sperm transfer is relatively rapid, beginning shortly after the initiation of copulation, and is essentially complete before the midpoint of copulation. Therefore, sperm transfer bears no simple relation to copulation duration. Because it would be difficult to manipulate the numbers of sperm transferred by manipulating copulation duration, methods used to study sperm displacement in other insect species are unlikely to be appropriate for D. melanogaster. We also investigated why males mate for more than twice the duration that appears to be necessary to complete sperm transfer. Experimental interruption of first matings indicated that the extra copulation time serves to delay female remating, rather than to increase that rate at which of offspring are sired before remating.
We have taken advantage of parallel instances of natural selection on body size in Drosophila melanogaster to investigate constraints and adaptation affecting wing shape. Using recently developed techniques for statistical shape analysis, we have examined variation in wing shape in similar body size clines on three continents. Gender-related shape differences were constant among all populations, suggesting that gender differences represent a developmental constraint on wing shape. In contrast, the underlying shape varied significantly between continents and shape change within each cline (i.e., between small and large body size populations) also varied between continents. Therefore, variation at these two levels presumably results from either drift or natural selection. Functional considerations suggest that shape variation between the continents is unlikely to be adaptive. However, cline-related shape change, which we show has a significant allometric component, may be adaptive. The overall range of wing shape variation, across a large range of wing size, is extremely small, and the possibility that wing shape is subject to stabilizing selection (or canalization) is discussed.
Surprisingly little is known about the genetic architecture of body size in natural populations of Drosophila melanogaster. Using both generation means and triple-test-cross analyses, we investigated the genetic architecture of wing size (an indicator of body size) and wing shape in a naturally occurring body size cline. For wing size, we found signi®cant epistatic genetic variance and evidence of past directional selection for increased body size. While wing shape also exhibits signi®cant epistatic genetic variance, there was no indication of directional selection, suggesting instead a history of optimizing selection. Our results support the idea that epistatic variance may be more common in natural populations than was once suspected. Also, our results suggest substantial directional selection on wing size but not shape.Keywords: directional selection, Drosophila melanogaster, epistasis, genetic architecture, wing shape, wing size. IntroductionBody size is of central importance in evolution and ecology, and it has been study extensively in both arti®cial and natural environments. Numerous allometric relationships between life history, physiological and behavioural traits and body size have been reported across species (e.g. Schmidt-Nielsen, 1984). Various trade-os aecting body size have been identi®ed (Stearns, 1992). Although arti®cial selection experiments provide numerous insights into aspects of the evolution of body size, the generality of the ®ndings is necessarily limited. Attempts to understand the how and why of evolution must eventually involve studies of natural populations.In Drosophila, in particular, surprisingly little is known about the quantitative genetics of body size of natural populations, apart from its high heritability (Coyne & Beecham, 1987; Prout & Barker, 1989; Ruiz et al., 1991; Thomas & Barker, 1993). Our intention in this study was to answer some additional and important questions regarding the evolution of body size in Drosophila melanogaster in a natural body-size cline. First, using wing area as a measure of body size, we questioned the importance of epistasis in population divergence. The answer is relevant not only to the shifting balance theory of evolution, but also to questions regarding the evolution of mating systems and conservation genetics (see Whitlock et al., 1995; Fenster et al., 1997). Secondly, what sort of selection (either directional or optimizing) is likely to be the predominant form of natural selection acting on the trait? Body size in D. melanogaster has an intermediate optimum value, largely determined by correlations with fecundity, development time and larval survival (Ro, 1981). Thus, although appearing to be under optimizing selection, the optimizing selection on body size is only apparent (Falconer, 1989): it is not clear what type of selection acts directly on genes determining body size within the constraints imposed by correlated characters.In addition to wing area, we also analysed wing shape. Although a number of investigators have documented ...
Sperm displacement has been the subject of a large number of evolutionary studies because of its e¡ects on relative male reproductive success. To understand better the evolutionary role of variation in sperm displacement ability (SDA), an obvious aim is to measure its heritability. In this paper, we show that a standard method used to measure the heritability of SDA can be misleading. First, we show that using conventional methods (based on counts of adult o¡spring of multiply mated females), SDA appears to be heritable. However, an examination of potentially confounding variables strongly suggests that this result is misleading, and that the heritable component is more likely to be pre-adult viability. Consequently, it is likely that there is little measurable heritable genetic variation for SDA in D. melanogaster. We conclude that, although conventional methods of measuring sperm displacement will usually be adequate for phenotypic measurements, greater care must be taken when measuring genetic variances.
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