Sexual dimorphism and allometry in the giant water strider Gigantometra gigas

University of Toronto, Toronto, Ontario, Canada
Canadian Journal of Zoology (Impact Factor: 1.3). 02/1999; 77(6). DOI: 10.1139/z99-071


This study describes sexual dimorphism in size (total body length and lengths of leg components) and in the allometric relationships between leg-component lengths and total body length in the giant water strider Gigantometra gigas (Heteroptera: Gerridae). Gigantometra gigas is the largest known gerrid, and has been previously described as monomorphic for body size. We compare our results with analogous data collected on Gerris buenoi, a species of more moderate size, where females are larger than males. Based on 94 specimens of G. gigas, we conclude that males are larger than females in all measured traits. This dimorphism was most spectacular in the leg components, which are 10-50% longer in males than in females. Males are generally more variable in size than females, and this is especially so for leg components. Allometric analysis suggests that total leg lengths (particularly middle and rear) increase at a much greater rate with body size in males than in females, therefore there is sexual dimorphism in allometries on the scale of that in the traits themselves. The relationship between middle and hind leg lengths is strong in both sexes, and appears to differ very little between the sexes or between G. gigas, and Ge. buenoi. These data suggest a constraint on this relationship, perhaps because of the biomechanics of locomotion. We propose that sexual selection acting on middle leg lengths in males explains both the increase and variance in middle leg length, and that hind leg length follows by correlated response.

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    • "V r h 2 = π body (Tseng and Rowe, 1999 "

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    • "Basally branching semi-aquatic insects can walk both on water and land, and share a common relative leg length plan, where T3-legs are longer than T2-legs, with their close relatives that are exclusively terrestrial (Fig. 1). Conversely derived lineages, which have specialized in water-surface locomotion through rowing, have evolved a novel adaptive morphology where the length of locomotory appendages has been reversed such that T2-legs are now longer than T3-legs (Fig. 1) (Andersen, 1982; Damgaard et al., 2005; Tseng and Rowe, 1999). Water striders use their T2-legs as propelling oars and their T3-legs as steering rudders (Andersen, 1982; Hu and Bush, 2010). "
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    ABSTRACT: Growth control scales morphological attributes and, therefore, provides a critical contribution to the evolution of adaptive traits. Yet, the genetic mechanisms underlying growth in the context of specific ecological adaptations are poorly understood. In water striders, adaptation to locomotion on the water surface is associated with allometric and functional changes in thoracic appendages, such that T2-legs, used as propelling oars, are longer than T3-legs, used as steering rudders. The Hox gene Ubx establishes this derived morphology by elongating T2-legs but shortening T3-legs. Using gene expression assays, RNAi knockdown, and comparative transcriptomics, we demonstrate that the evolution of water surface rowing as a novel means of locomotion is associated with the evolution of a dose-dependent promoting-repressing effect of Ubx on leg growth. In the water strider Limnoporus dissortis, T3-legs express six to seven times higher levels of Ubx compared to T2-legs. Ubx RNAi shortens T2-legs and the severity of this phenotype increases with increased depletion of Ubx protein. Conversely, Ubx RNAi lengthens T3-legs but this phenotype is partially rescued when Ubx protein is further depleted. This dose-dependent effect of Ubx on leg growth is absent in non-rowing relatives that retain the ancestral relative leg length. We also show that the spatial patterns of expression of dpp, wg, hh, egfr, dll, exd, hth, and dac are unchanged in Ubx RNAi treatments. This indicates that the dose-dependent opposite effect of Ubx on T2- and T3-legs operates without any apparent effect on the spatial expression of major leg patterning genes. Our data suggest that scaling of adaptive allometries can evolve through changes in the levels of expression of Hox proteins early during ontogeny, and in the sensitivity of the tissues that express them, without any major effects on pattern formation.
    Developmental Biology 05/2014; 392(2). DOI:10.1016/j.ydbio.2014.05.021 · 3.55 Impact Factor
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    • "The ventral fin is hidden from conspecific observers in this land fish and its size is subsequently not visible for assessment by mates or rivals. Furthermore, the ventral fin may exhibit some positive allometry because of the biomechanical demands of swimming [83], which would also be true for the dorsal fin and perhaps even the head crest as well (e.g., if it functioned as a rudder). That is, although the Pacific leaping blenny rarely, if ever, ventures into the water, the dorsal fin or head crest might still exhibit positive allometry, not because of contemporary sexual selection, but because of an evolutionary history of swimming in marine ancestors (e.g., historical natural selection on swimming performance). "
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    ABSTRACT: Background Divergence between populations in reproductively important features is often vital for speciation. Many studies attempt to identify the cause of population differentiation in phenotype through the study of a specific selection pressure. Holistic studies that consider the interaction of several contrasting forms of selection are more rare. Most studies also fail to consider the history of connectivity among populations and the potential for genetic drift or gene flow to facilitate or limit phenotypic divergence. We examined the interacting effects of natural selection, sexual selection and the history of connectivity on phenotypic differentiation among five populations of the Pacific leaping blenny (Alticus arnoldorum), a land fish endemic to the island of Guam. Results We found key differences among populations in two male ornaments—the size of a prominent head crest and conspicuousness of a coloured dorsal fin—that reflected a trade-off between the intensity of sexual selection (male biased sex ratios) and natural selection (exposure to predators). This differentiation in ornamentation has occurred despite evidence suggesting extensive gene flow among populations, which implies that the change in ornament expression has been recent (and potentially plastic). Conclusions Our study provides an early snapshot of divergence in reproductively important features that, regardless of whether it reflects genetic or plastic changes in phenotype, could ultimately form a reproductive barrier among populations.
    BMC Evolutionary Biology 05/2014; 14(1):97. DOI:10.1186/1471-2148-14-97 · 3.37 Impact Factor
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