Comparative Analysis of Morphological Traits Among Drosophila Melanogaster and D. Simulans: Genetic Variability, Clines and Phenotypic Plasticity

Abstract

The two sibling cosmopolitan species, Drosophila melanogaster and D. simulans, are able to proliferate under very different climatic conditions. This has resulted in local adaptations, which are often arranged in latitudinal clines. Such clines are documented for body weight, wing and thorax length, sternopleural and abdominal bristle number, ovariole number and thoracic pigmentation. The overall magnitude of geographical differentiation is, however, much less in D. simulans than in D. melanogaster, and latitudinal clines are less pronounced. The fact that natural populations live under different climates raises the problem of interaction between temperature and phenotype. The reaction norms of morphometrical traits have been investigated as a function of growth temperature. The shapes of the response curves vary according to the investigated trait. They are generally curvilinear and can be described by calculating characteristic values after polynomial adjustments. For a given trait, the reaction norms of the two species are similar in their shape, although some significant differences may be observed. Within each species, significant differences are also observed between geographic populations: reaction norms are not parallel and the divergence is better marked when more distant populations (e.g., temperate and tropical) are compared. It thus appears that besides mean trait value, phenotypic plasticity is also a target of natural selection. A specific analysis of wing shape variation according to growth temperature was also undertaken. Reaction norms with different shapes may be observed in various parts of the wing: the major effect is found between the basis and the tip of the wing, but in a similar way in the two species. By contrast, some ratios, called wing indices by taxonomists, may exhibit completely different reaction norms in the two species. For a single developmental temperature (25 degrees C) the phenotypic variability of morphometrical traits is generally similar in the two species, and also the genetic variability, estimated by the intraclass correlation. A difference exists, however, for the ovariole number which is less variable in D. simulans. Variance parameters may vary according to growth temperature, and a detailed analysis was made on wing dimensions. An increase of environmental variability at extreme, heat or cold temperatures, has been found in both species. Opposite trends were, however, observed for the genetic variability: a maximum heritability in D. simulans at middle temperatures, corresponding to a minimum heritability in D. melanogaster. Whether such a difference exists for other traits and in other populations deserves further investigations. In conclusion, morphometrical analyses reveal a large amount of significant differences which may be related to speciation and to the divergence of ecological niches. Within each species, numerous geographic variations are also observed which, in most cases, reflect some kinds of climatic adaptation.

Rapid response to abiotic and biotic factors controls population growth of two invasive drosophilids (Diptera) in the Brazilian Savanna

“…Reinforcing the arguments presented above, a higher predictability of the biology and dynamics of D. simulans, compared with Z. indianus, has been reported in worldwide studies (e.g. Karan et al 2000;Gibert et al 2004;David et al 2006;Yassin et al 2009;Commar et al 2012;van Heerwaarden et al 2012).…”

“…We made an additional change to the model for Z. indianus: we changed the cause-effect positive relationship mean temperature tem ? body size siz to the covariance relation tem $ siz because previous studies have demonstrated an inverse relationship for these variables (Nunney and Cheung 1997;Gibert et al 2004). The goodness of fit of the model was unchanged.…”

“…Moreover, larval density has been demonstrated to negatively affect adult body size (James et al 1997), which in turn is positively correlated with longevity and with adult survivorship rates (Santos et al 1992;Klingenberg and Spence 1997) and fecundity (Klingenberg and Spence 1997;Creighton 2005). Finally, water availability affects ovary development (Wheeler 1996;Papaj 2000), activity rates (David et al 1983) and both adult and pre-adult survivorship rates (David et al 1983), whereas temperature is inversely correlated with body size and activity rate (David et al 1983;Nunney and Cheung 1997;Hodin and Riddiford 2000;Gibert et al 2004).…”

Genetic architecture and phenotypic plasticity of thermally-regulated traits in an eruptive species, Dendroctonus ponderosae

“…Latitudinal clines in phenotypic traits within and among insect species are widespread (Nylin et al 1989;Gilchrist and Partridge 1999;Fox et al 2004;Gibert et al 2004). Because critical physiological processes in insects are thermally regulated (Taylor 1981), temperature is considered a major environmental factor influencing variability in morphological and life history traits along clines.…”

Counting in oogenesis

“…Studies of laboratory populations have revealed that an intraspecific maximum ovariole number is attained with optimal larval nutrition at intermediate temperatures (Delpuech et al 1995;Thomas-Orillard 1984). In studies of natural populations of D. melanogaster and D. simulans, the ovariole number has been found to vary along a latitudinal cline, distinguishing populations within each species (Gibert et al 2004). Both species show a similar clinal variation in the ovariole number, establishing that different species respond in similar ways to environmental influences.…”