Body size and developmental time in Drosophila: Is there a trade-off?

CARREIRA VALERIA; MENSCH JULIÁN; FANARA JUAN JOSÉ

Ribeirão Preto, São Paulo, Brasil.

Simposio; V Simpósio de Ecologia, Genética e Evolução de Drosophila.CARREIRA; 2007

Sociedad Brasilera de Genética

Theory concerning life histories evolution consider the existence of functional constraints in the form of trade-offs between fitness related traits usually studied by phenotypic correlations. However, only genetic correlations give the information necessary to establish the existence of trade-offs and, therefore, functional constraints should be considered as genetic correlations imposed by pleiotropy. Developmental time (DT) and adult body size (BS) are two important traits in Drosophila life history which seems to be involved in a trade-off. In D. melanogaster, some studies showed that artificial selection for large BS results in an increase in DT with a reduction in larval viability under high densities. Conversely, selection for fast development reduces adult body weight and, hence, lifetime fecundity. These results suggest the existence of loci exerting antagonistic effects on both traits. So, it should be demonstrated that candidate genes generating quantitative variation in one of those traits also exert an effect on the other in the direction predicted by the trade-off. However, as in Drosophila there are two periods of larval growth, a flexible precritical one and a fixed postcritical one, the relationships between DT, BS and larval survival may not be predictable as suggested by some inconsistent evidence. Despite the existence of many studies concerning the mentioned trade-off, there are no studies identifing genes affecting both traits simultaneously. Therefore, our goal is to shed some light on this problem searching for candidate genes that contribute to BS and DT in Drosophila melanogaster. In this sense, a loss-of-function screening using a P-element insertional mutagenesis methodology was employed to identify candidate genes affecting both traits. The screening consist in finding phenotypic differences between P-element insertion lines and a co-isogenic control line P-element insertion free. In order to identify the mutated genes, nucleotide sequences flanking the P-element insertion were employed as “in silico” probes to search homologous regions in the D. melanogaster genome. Flies corresponding to 112 independent co-isogenic lines were raised in a controlled environment. DT was estimated as the time elapsed since the transfer of first instar larvae to the vials until adult emergence. Face width, head width, thorax length and wing size (estimated by centroid size) were used as estimators of BS. Sexes were analysed separatelly due to the high sexual dimorphism for BS. We found negative genetic correlations between DT and some BS related traits, a finding that clearly contradicts what we would expect if there were antagonistic pleiotropy. Also, only in 20% of the lines, both traits were significantly affected by the mutation and it generally caused an enlargement of DT and a reduction of body size. Based on this results, we discuss the relevance of the candidate genes identified in the context of the mentioned trade-off.