Deciphering the Evolutionary History and Developmental Mechanisms of a Complex Sexual Ornament: The Abdominal Appendages of Sepsidae (Diptera)

Abstract

Male abdomen appendages are a novel trait found within Sepsidae (Diptera). Here we demonstrate that they are likely to have evolved once, were lost three times, and then secondarily gained in one lineage. The developmental basis of these appendages was investigated by counting the number of histoblast cells in each abdominal segment in four species: two that represented the initial instance of appendage evolution, one that has secondarily gained appendages, and one species that did not have appendages.Males of all species with appendages have elevated cell counts for the fourth segment, which gives rise to the appendages. In Perochaeta dikowi, which reacquired the trait, the females also have elevated cell count on the fourth segment despite the fact that females do not develop appendages. The species without appendages has similar cell counts in all segments regardless of sex.These results suggest that the basis for appendage development is shared in males across all species, but the sexual dimorphism is regulated differently in P. dikowi. K E Y W O R D S :Appendage, developmental constraint, innovation, parallel evolution, Sepsidae.

Hexapoda: Comparative Aspects of Later Embryogenesis and Metamorphosis

“…Abdominal appendages have also reevolved in the nymphs of some aquatic insects, in adult male sepsid fl ies ( Fig. 3.21A ; Bowsher et al 2013 ) and in a recently discovered bug (Hoch et al 2014 ). The embryos of many insects develop a transient appendage pair on the fi rst abdominal segment (A1).…”

“…These are a rare example of abdominal appendages in an adult holometabolous insect. Like true appendages, these structures articulate with the body wall via a joint and are mobile in the species in which they are best developed, such as Themira biloba (Bowsher et al 2013 ). Unlike appendages, they develop from histoblast nests (which give rise to the abdominal epidermis, but not genitalia or thoracic appendages, in fl ies; Bowsher and Nijhout 2007 ).…”

Grand challenges in evolutionary developmental biology

“…Sucena et al (2014) have investigated the potential reversion from long to short germband in the braconid wasps, which ancestrally display long germband development (itself a derived condition within insects) but short germband development has secondarily evolved in the polyembryonic species Macrocentrus cingulum, suggesting that evolutionary change in germband size in insects is a reversible process. Similarly, the peculiar abdominal appendages evolved in the male sepsid flies have been shown by Bowsher et al (2013) to have likely appeared only once, but within the family these appendages have been lost three times, and secondarily re-evolved in one lineage.…”

Using seemingly unnecessary illustrations to improve the diagnostic usefulness of descriptions in taxonomy–a case study on Perochaeta orientalis (Diptera, Sepsidae)

“…Sternite 4 heavily modified into paired moveable appendages [Fig. 1F; see Bowsher et al (2013) for a discussion on the evolution of the appendages and Fig. 5 for sternite appendage illustrations of other Perochaeta ]: largely desclerotized except for anterior margin as well as two rectangular regions laterally off the median.…”