Phylogenetics of Eggshell Morphogenesis in Antheraea (Lepidoptera: Saturniidae): Unique Origin and Repeated Reduction of the Aeropyle Crown

Abstract

Integrated phylogenetic and developmental analyses should enhance our understanding of morphological evolution and thereby improve systematists' ability to utilize morphological characters, but case studies are few. The eggshell (chorion) of Lepidoptera (Insecta) has proven especially tractable experimentally for such analyses because its morphogenesis proceeds by extracellular assembly of proteins. This study focuses on a morphological novelty, the aeropyle crown, that arises at the end of choriogenesis in the wild silkmoth genus Antheraea. Aeropyle crowns are cylindrical projections, ending in prominent prongs, that surround the openings of breathing tubes (aeropyle channels) traversing the chorion. They occur over the entire egg surface in some species, are localized to a circumferential band in many others, and in some are missing entirely, thus exhibiting variation typical of discrete characters analyzed in morphological phylogenetics. Seeking an integrated developmental-phylogenetic view, we first survey aeropyle crown variation broadly across Antheraea and related genera. We then map these observations onto a robust phylogeny, based on three nuclear genes, to test the adequacy of character codings for aeropyle crown variation and to estimate the frequency and direction of change in those characters. Thirdly, we draw on previous studies of choriogenesis, supplemented by new data on gene expression, to hypothesize developmental-genetic bases for the inferred chorion character transformations. Aeropyle crowns are inferred to arise just once, in the ancestor of Antheraea, but to undergo four or more subsequent reductions without regain, a pattern consistent with Dollo's Law. Spatial distribution shows an analogous trend, though less clear-cut, toward reduction of coverage by aeropyle crowns. These trends suggest either that there is little or no natural selection on the details of the aeropyle crown structure or that evolution toward functional optima is ongoing, although no direct evidence exists for either. Genetic, biochemical, and microscopy studies point to at least two developmental changes underlying the origin of the aeropyle crown, namely, reinitiation of deposition of chorionic lamellae after the end of normal choriogenesis (i.e., heterochrony), and sharply increased production of underlying "filler" proteins that push the nascent final lamellae upward to form the crown (i.e., heteroposy). Identification of a unique putative cis-regulatory element shared by unrelated genes involved in aeropyle crown formation suggests a possible simple mechanism for repeated evolutionary reduction and spatial restriction of aeropyle crowns.

Morphology of the preimaginal stages of the African emperor moth Bunaeopsis licharbas (Maassen and Weyding): Phylogenetically informative characters within the Saturniinae (Lepidoptera: Saturniidae)

“…According to these authors, such structures would have appeared once (within the Saturniinae) in the genus Antheraea; it would have been subsequently reduced within the genus on several independent occasions and to different extents. Interestingly, the situation in B. aurantiaca fits the character state A 0 ''rudimentary swellings around aeropyle opening'' defined for a few Antheraea species by Regier et al (2005;see their Regier et al (2005;p. 260).…”

“…Nevertheless, as far as we know, the egg exochorion of a member of the Bunaeini never was figured and described until now. The general appearance of cells, cell walls, and aeropyles in Bunaeopsis licharbas is very similar to that in several members of the tribes Saturniini and Attacini (for instance in Graellsia isabellae, Antherina suraka, Archaeoattacus edwardsii, and A. staudingeri, see Figures 7A, E, H, and I in Regier et al (2005), respectively). The only possibly significant feature observed in the Bunaeini B. licharbas is the very irregular shape of the cells, with a variable number of edges, on the sides of the eggs.…”

“…71) thinner in B. aurantiaca. The observation of a slightly elevated aeropyle opening in B. aurantiaca is interesting with respect to the recent findings by Regier et al (2005) about the evolution of the aeropyle crown within the Saturniinae. According to these authors, such structures would have appeared once (within the Saturniinae) in the genus Antheraea; it would have been subsequently reduced within the genus on several independent occasions and to different extents.…”

“…The use of egg characters for phylogenetic reconstruction was shown by Garcia-Barros and Martin (1995) to be subject to a high level of homoplasy, suggesting, along with the results of Regier et al (2005), a high selective pressure on this stage. On the other hand, some interesting similarities between the eggs of the Attacini Samia abrerai (Paukstadt and Paukstadt, 2000c) and Attacus spp.…”

“…The study carried out by Pljushch and Dolinskaya (2001) was the first to compare chorion morphology among a small set of three species belonging to the tribe Saturniini. Recently, among research on developmental issues in the formation of Lepidoptera eggshell, Regier et al (2005) focused on the phylogenetics of eggshell morphogenesis in the genus Antheraea within the Saturniini tribe; these authors also provided hitherto unpublished SEM figures of the eggshell of several species of Saturniinae within the tribes Saturniini and Attacini. Nevertheless, as far as we know, the egg exochorion of a member of the Bunaeini never was figured and described until now.…”