Kevin Fowler

Do sexual ornaments demonstrate heightened condition-dependent expression as predicted by the handicap hypothesis?

The handicap hypothesis of sexual selection predicts that sexual ornaments have evolved heightened condition–dependent expression. The prediction has only recently been subject to experimental investigation. Many of the experiments are of limited value as they: (i) fail to compare condition dependence in sexual ornaments with suitable non–sexual trait controls; (ii) do not adequately account for body size variation; and (iii) typically consider no stress and extreme stress manipulations rather than a range of stresses similar to those experienced in nature. There is also a dearth of experimental studies investigating the genetic basis of condition dependence. Despite the common claim that sexual ornaments are condition–dependent, the unexpected conclusion from our literature review is that there is little support from well–designed experiments.

Evolution and development of body size and cell size in Drosophila melanogaster in response to temperature

We examined the evolutionary and developmental responses of body size to temperature in Drosophila melanogaster, using replicated lines of flies that had been allowed to evolve for 5 yr at 25°C or at 16.5°C. Development and evolution at the lower temperature both resulted in higher thorax length and wing area. The evolutionary effect of temperature on wing area was entirely a consequence of an increase in cell area. The developmental response was mainly attributable to an increase in cell area, with a small effect on cell number in males. Given its similarity to the evolutionary response, the increase in body size and cell size resulting from development at low temperature may be a case of adaptive phenotypic plasticity. The pattern of plasticity did not evolve in response to temperature for any of the traits. The selective advantage of the evolutionary and developmental responses to temperature is obscure and remains a major challenge for future work.

DIRECT AND CORRELATED RESPONSES TO SELECTION ON AGE AT REPRODUCTION IN DROSOPHILA MELANOGASTER

Aging may be a consequence of mutation accumulation or of negative pleiotropic correlations between performance late and earlier in the lifespan. This study used artificial selection on flies derived from two different base stocks to produce “young” and “old” lines, propagated by breeding from young and old adults respectively. Virgin and mated adults of both sexes from the “old” lines lived longer than “young” line flies. “Young” and “old” mated females did not differ in fecundity or fertility early in the lifespan, but “old” line females had higher fecundity and fertility late in life. The results therefore suggested either that the response to selection had revealed the effect of mutation accumulation, or that pleiotropy involving characters other than early fecundity must have been involved. Development time from egg to adult was longer in the “old” lines. Competition of selected line larvae from one base stock against mutant marked larvae from the same base stock revealed that, at a wide range of larval densities, “old” line larvae showed lower survival rates than “young” line larvae. Thorax length and wet weight were significantly greater in the “old” line flies from one base stock. The results may imply that the selection regime in the “old” lines favored extended growth during development to produce a more durable adult soma, despite the cost in increased larval mortality and delayed reproduction, because the potential reproductive benefits later in life were increased. However, the differences between larvae from “old” and “young” lines could also be attributable to density differences, and this possibility needs systematic investigation.

Condition-dependent signalling of genetic variation in stalk-eyed flies

Handicap models of sexual selection predict that male sexual ornaments have strong condition-dependent expression and this allows females to evaluate male genetic quality. A number of previous experiments have demonstrated heightened condition-dependence of sexual ornaments in response to environmental stress. Here we show that genetic variation underlies the response to environmental stress (variable food quality) of a sexual ornament (male eye span) in the stalk-eyed fly Cyrtodiopsis dalmanni. Some male genotypes develop large eye span under all conditions, whereas other genotypes progressively reduce eye span as conditions deteriorate. Several non-sexual traits (female eye span, male and female wing length) also show genetic variation in condition-dependent expression, but their genetic response is entirely explained by scaling with body size. In contrast, the male sexual ornament still reveals genetic variation in the response to environmental stress after accounting for differences in body size. These results strongly support the hypothesis that female mate choice yields genetic benefits for offspring.

Effects of egg-production and of exposure to males on female survival in Drosophila melanogaster

Abstract Female Drosophila melanogaster whose rate of egg-laying was lowered by removal of a suitable substrate or additionally by reduction in protein intake had longer lifespans than control females. The association between lifespan and egg-laying rate could not be accounted for in terms of variation in either exposure to males or nutritional status. Females continuously exposed to males lived less long than intermittently exposed females, and this longevety difference could not be explained in terms of differences in either rate of egg-production or egg-hatchability.

What does sexual trait FA tell us about stress

There is widespread acceptance that fluctuating asymmetry (FA) increases under environmental and genetic stress. Fluctuating asymmetry in sexual traits is thought to be particularly sensitive to stress and to reflect genetic and phenotypic quality. Recent experimental studies show that the relationship between FA and stress is inconsistent, and there is little evidence that sexual traits are especially responsive to stress.

CONDITION DEPENDENCE OF SEXUAL ORNAMENT SIZE AND VARIATION IN THE STALK-EYED FLY CYRTODIOPSIS DALMANNI (DIPTERA: DIOPSIDAE)

Abstract We used the stalk‐eyed fly Cyrtodiopsis dalmanni to examine predictions made by condition‐dependent handicap models of sexual selection. Condition was experimentally varied by manipulation of larval food availability. Cyrtodiopsis dalmanni is a highly dimorphic species exhibiting strong sexual selection, and the male sexual ornament (exaggerated eyespan) showed strong condition‐dependent expression relative to the homologous trait in females and nonsexual traits. Male eyespan also showed a great increase in standardized variance under stress, unlike nonsexual traits. The inflated variance of the male ornament was primarily attributable to condition‐dependent (but body‐size‐independent) increase in variance. Thus, evaluation of male eyespan allows females to gain additional information about male condition over and above that given by body size. These findings accord well with condition‐dependent handicap models of sexual selection.

Non-mating costs of exposure to males in female Drosophila melanogaster.

Abstract Female D. melanogaster exposed to males have shorter lifespans than females not so exposed, in the absence of differences in egg-production and egg-fertility. It is here shown that part of this reduction is caused by events prior to mating. Both non-sexual effects and some consequence of male sexual activities appear to be involved. The mechanism by which non-mating exposure reduces lifespan appears to depend upon the mating status of the females.

Responses and correlated responses to artificial selection on thorax length in Drosophila melanogaster .

Two sets of four replicate lines of Drosophila melanogaster were selected for large and small thorax with controls. F, progeny of crosses between the selected lines within each size category showed (a) a reduction in preadult viability in large lines relative to control and small lines when they were cultured at medium or high density in competition with a standard mutant marked competitor stock, and (b) an increase in larval development time in large lines relative to control and small lines. Natural selection for increased body size in adults may therefore be opposed by adverse effects on larval viability. The results are discussed in terms of the developmental mechanisms probably responsible for the change in body size. The preadult survival of the large and control lines was measured at three different temperatures, and there was no evidence for a significant interaction between size and temperature. The observed evolutionary increase in body size in response to reduced temperature in Drosophila must therefore involve either different genes from those subject to selection for size at a single temperature, or a fitness component other than preadult survival. There was no significant asymmetry in response to selection, and thorax length showed heterosis in crosses between the selected lines.

RAPID LABORATORY EVOLUTION OF ADULT LIFE-HISTORY TRAITS IN DROSOPHILA MELANOGASTER IN RESPONSE TO TEMPERATURE

Three replicate lines of Drosophila melanogaster were cultured at each of two temperatures (16.5°C and 25°C) in population cages for 4 yr. The lifespans of both sexes and the fecundity and fertility of the females were then measured at both experimental temperatures. The characters showed evidence of adaptation; flies of both sexes from each selection regime showed higher longevity, and females showed higher fecundity and fertility, than flies from the other selection regime when they were tested at the experimental temperature at which they had evolved. Calculation of intrinsic rates of increase under different assumptions about the rate of population increase showed that the difference between the lines from the two selection regimes became less the higher the rate of population increase, because the lines were more similar in early adulthood than they were later. Despite the increased adaptation of the low‐temperature lines to the low temperature, like the high temperature lines they produced progeny at a higher rate at the higher temperature. The lines may have independently evolved adaptations to their respective thermal regimes during the experiment, or there may have been a trade‐off between adaptation to the two temperatures, or mutation pressure may have lowered adaptation to the temperature that the flies no longer encountered.