Abstract
BackgroundThe males of some sepsid species (Sepsidae: Diptera) have abdominal appendages that are remarkable in several ways. They are sexually dimorphic, have a complex evolutionary history of gain and loss, and can be jointed and thus highly mobile. The sternite brushes are used extensively in complex courtship behaviors that differ considerably between species and during mating. The abdominal appendages have a novel developmental pathway developing from histoblast nests rather than imaginal discs.ResultsWe focus on the evolution of cell number, nest area, and segment length in both sexes to understand how this tissue relates to the formation of novel abdominal appendages. We map histoblast nest size of wandering-phase larvae of 17 species across 10 genera to a phylogenetic tree of Sepsidae and demonstrate that abdominal appendages require significant increases of histoblast nest size and cell number in most species while one species produces small appendages even without such modifications. In species with particularly large appendages, not only the nests on the fourth, but nests in neighboring segments are enlarged (Themira biloba, Themira putris). The loss of abdominal appendages corresponds to the loss of an enlarged fourth histoblast nest, although one species showed an exception to this pattern. One species that constitutes an independent origin of abdominal appendages (Perochaeta dikowi) uses an unusual developmental mechanism in that the histoblast nest sizes are not sexually dimorphic.ConclusionsThe surprisingly high diversity in histoblast size and degree of sexual dimorphism suggests that the developmental mechanism used for abdominal appendage formation in sepsids is highly adaptable. The presence of appendages usually correlate with increased histoblast cell number and in most cases appendage loss results in a return to ancestral histoblast morphology. However, we also identify several exceptions that indicate the abdominal appendages have a malleable developmental origin that is responsive to selection.
Highlights
The males of some sepsid species (Sepsidae: Diptera) have abdominal appendages that are remarkable in several ways
The objective of this study was to determine if histoblast nest morphology between sexes and segments was consistent across the evolutionary history of gains and losses of the appendage
A previous study that included four species determined that histoblast cell counts have three distinct patterns which correspond to adult abdominal appendage morphology with an increase in histoblast cell number correlating with the presence of the appendage [14]
Summary
The males of some sepsid species (Sepsidae: Diptera) have abdominal appendages that are remarkable in several ways. They are sexually dimorphic, have a complex evolutionary history of gain and loss, and can be jointed and highly mobile. The abdominal appendage is an evolutionarily novel structure with no known homology within Diptera [11–13], and has a complex evolutionary history of gain and loss [14]. The abdominal appendages have a novel developmental origin from histoblast nests, which are clusters of imaginal cells that lack the complex three-dimensional organization of imaginal discs which produce limbs and other adult morphological structures. Males in some species of sepsids have enlarged histoblast nests in the appendage-producing segment. An exception is Perochaeta dikowi, an appendage-bearing species with enlarged histoblast nests in males and females [14]
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