Leg length and bristle density both necessary for water surface locomotion are genetically correlated in water striders

Access to hitherto unexploited ecological opportunities is associated with
phenotypic evolution and often results in significant lineage
diversification. Yet, our understanding of the mechanisms underlying such
adaptive traits remains limited.  Water striders have been able
to exploit the water-air interface, primarily facilitated by changes in
the density of hydrophobic bristles and a significant increase in leg
length. These two traits are functionally correlated and are both
necessary for generating efficient locomotion on the water surface.
Whether bristle density and leg length have any cellular or developmental
genetic mechanisms in common is unknown. Here, we combine comparative
genomics and transcriptomics with functional RNAi assays to examine the
developmental genetic and cellular mechanisms underlying the patterning of
the bristles and the legs in Gerris buenoi and Mesovelia mulsanti, two
species of water striders. We found that two gene duplication events in
the genes beadex and taxi led to a functional expansion of the paralogs to
affect bristle density and leg length. We also identified genes for which
no function in bristle development has been previously described in other
insects. Interestingly, most of these genes play a dual role in regulating
bristle development and leg length. In addition, these genes play a role
in regulating cell division. This result suggests that cell division may
be a common mechanism through which these genes can simultaneously
regulate leg length and bristle density. We propose that pleiotropy, by
which gene function affects the development of multiple traits, may play a
prominent role in facilitating access to unexploited ecological
opportunities and species diversification.