Abstract
BackgroundEstablishment of geographic morph frequency clines is difficult to explain in organisms with limited gene flow. Balancing selection, such as negative frequency-dependent selection (NFDS), is instead suggested to establish a morph frequency cline on a geographic scale at least theoretically. Here we tested whether a large-scale smooth cline in morph frequency is established by NFDS in the female-dimorphic damselfly, Ischnura senegalensis, where andromorphs and gynomorphs are maintained by NFDS.ResultsWe found a large-scale latitudinal cline in the morph frequency: andromorph frequency ranged from 0.05 (South) to 0.79 (North). Based on the empirical data on the numbers of eggs, the number of ovariole, abdomen length and latitude, the potential fitness of andromorphs was estimated to be lower than that of gynomorphs in the south, and higher in the north, suggesting the gene-by-environment interaction. From the morph-specific latitudinal cline in potential fitness, the frequency of andromorphs was expected to shift from 0 to 1 without NFDS, because a morph with higher potential fitness wins completely and the two morphs will switch at some point. In contrast, NFDS led to the coexistence of two morphs with different potential fitness in a certain geographic range along latitude due to rare morph advantage, and resulted in a smooth geographic cline of morph frequency.ConclusionOur results provide suggestive evidence that the combination of NFDS and gene-by-environment interaction, i.e., multi-selection pressure on color morphs, can explain the geographic cline in morph frequency in the current system.
Highlights
Establishment of geographic morph frequency clines is difficult to explain in organisms with limited gene flow
Morph-specific latitudinal cline in potential fitness We here estimated the potential fitness of andromorphs and gynomorphs along latitude, based on the empirical data on the length of abdomen, the number of ovariole, egg size and latitude, assuming the gene-by-environment interaction
We showed that the current latitudinal cline in morph frequency is well explained by the combination of negative frequency-dependent selection (NFDS) [34] and the differing potential fitness between morphs without the effect of gene flow
Summary
Establishment of geographic morph frequency clines is difficult to explain in organisms with limited gene flow. Geographic clines in quantitative traits caused by environmental gradients have been reported for many species [2,5] Those are typically smooth due to gradual changes in selection in relation to environmental factors such as temperature [2]. Clines are observed in morph (or allele) frequencies in species with genetic polymorphisms in both natural and laboratory systems [3,6] In these cases, the fitness advantage of each morph differentially changes with environmental gradient (i.e., gene-by-environment interaction), and reverses across an equilibrium (balancing) point, where each phenotype has equal fitness [6]. Theoretically, morph frequency is expected to show a steep cline (stepwise pattern) across an equilibrium point in the absence of other evolutionary forces antagonistic to local selection [1,6]
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