Molecules versus morphology: the detection of selection acting on morphological characters along a cline in Drosophila melanogaster

Abstract

This work examines the nature of north-south clinal variation in morphological characters in Drosophila melanogaster. Isofemale lines were established from flies collected along a transect extending from Winnipeg, Manitoba (Canada) to Tampa Bay, Florida (U.S.A.). Offspring from different lines within each location were then cultured under standardized conditions and used to examine phenotypic variation in seven morphological characters along the dine. In addition, allozyme variation at seven polymorphic loci was examined for the same set of clinal populations. Scutellum length and wing length show the strongest clinal trends. Clinal variation is nonmonotomc, with larger flies in the middle latitudes and smaller flies in the north and south. This result contrasts with other studies which have shown monotonic dines. Patterns of population subdivision were different for the different characters. This implies that there are different selective forces acting on the different morphological characters. Based on a comparison of morphological and molecular population subdivision for adjacent populations it is inferred that natural selection is operating to maintain a high level of population subdivision for wing width and the first principal component between one of the sets of populations. A combined approach using molecules and morphology may provide an alternative to retrospective selection analysis for detecting selection in nature.

Population genetic structure in a Mediterranean pine (Pinus pinaster Ait.): a comparison of allozyme markers and quantitative traits

“…The interaction effect (␣␤ ij ) reflects the lack of proportionality between allozyme (F ST *) and quantitative (Q ST *) differentiation. If the two measures of differentiation are proportional over the set of populations compared, the absence of selection acting on the quantitative trait is assumed and incremental effects are attributed to the different kind of data analysed (Long and Singh, 1995). Great differences in patterns of population subdivision were present for the traits analysed.…”

“…Long and Singh (1995) showed that the choice of enzyme systems may result in a difference between quantitative and allozyme population subdivision, which is not because of the action of natural selection. To avoid this pitfall they proposed to compare pairs of populations.…”

“…Values of F ST and Q ST were angular transformed (y* = arcsine (y) 1/2 ) to approach normality. In order to test the deviation of quantitative (Q ST *) and allozyme (F ST *) differentiation avoiding potential bias due to selection of molecular markers, the following linear model was fitted for each quantitative trait (Long and Singh, 1995):…”

“…Although extensive gene flow has been estimated in P. pinaster using molecular markers (Nm = 3.02; Salvador et al, 2000), high quantitative differ-entiation is detected in this study. Long and Singh (1995) and Lynch (1995) showed that in nature two populations can experience high gene flow and be different for a quantitative trait. As shown in other species (Bossart and Scriber, 1995), environmental heterogeneity and genotype-by-environment interaction seem to have a major role in quantitative differentiation when gene flow is high.…”

“…The combination of quantitative and molecular measures of variation allows one to test empirically hypotheses relating to different modes of phenotypic evolution in subdivided populations. Some recent papers dealing with Drosophila (Prout and Barker, 1993;Long and Singh, 1995), Daphnia (Spitze, 1993;Lynch et al, 1999), Clarkia (Podolsky and Holtsford, 1995), Medicago (Bonnin et al, 1996), Scabiosa (Waldmann and Andersson, 1998), Arabidopsis (Kuittinen et al, 1997), Salix viminalis (Lascoux et al, 1996), Quercus petraea (Kremer et al, 1997) and Pinus contorta ssp. latifolia (Yang et al, 1996) have reported joint estimates of quantitative and molecular traits by means of F-statistics.…”