Quezada- Euán J.J.G., Sanabria-Urbán S., Smith C.,Cueva del Castillo R. (2019). Patterns of sexual size dimorphism in stingless bees: Testing Rensch’s rule and potential causes in highly eusocial bees (Hymenoptera: Apidae, Meliponini). Ecology and Evolution 9 (5): 2688-2698.
Abstract
Eusocial insects offer a unique opportunity to analyze
the evolution of body size differences between sexes in relation to
social environment. The workers, being sterile females, are not subject
to selection for reproductive function providing a natural control for
parsing the effects of selection on reproductive function (i.e., sexual
and fecundity selection) from other kinds of natural selection. Patterns
of sexual size dimorphism (SSD) and testing of Rensch's rule
controlling for phylogenetic effects were analyzed in the Meliponini or
stingless bees. Theory predicts that queens may exhibit higher selection
for fecundity in eusocial taxa, but contrary to this, we found mixed
patterns of SSD in Meliponini. Non‐Melipona species generally have a female‐biased SSD, while all analyzed species of Melipona showed
a male‐biased SSD, indicating that the direction and magnitude of the
selective pressures do not operate in the same way for all members of
this taxon. The phylogenetic regressions revealed that the rate of
divergence has not differed between the two castes of females and the
males, that is, stingless bees do not seem to follow Rensch's rule (a
slope >1), adding this highly eusocial taxon to the various solitary
insect taxa not conforming with it. Noteworthy, when Melipona was
removed from the analysis, the phylogenetic regressions for the thorax
width of males on queens had a slope significantly smaller than 1,
suggesting that the evolutionary divergence has been larger in queens
than males, and could be explained by stronger selection on female
fecundity only in non‐Melipona species. Our results in the
stingless bees question the classical explanation of female‐biased SSD
via fecundity and provide a first evidence of a more complex
determination of SSD in highly eusocial species. We suggest that in
highly eusocial taxa, additional selection mechanisms, possibly related
to individual and colonial interests, could influence the evolution of
environmentally determined traits such as body size.
