Insights into the Development and Evolution of Exaggerated Traits Using De Novo Transcriptomes of Two Species of Horned Scarab Beetles

Abstract

Scarab beetles exhibit an astonishing variety of rigid exo-skeletal outgrowths, known as “horns”. These traits are often sexually dimorphic and vary dramatically across species in size, shape, location, and allometry with body size. In many species, the horn exhibits disproportionate growth resulting in an exaggerated allometric relationship with body size, as compared to other traits, such as wings, that grow proportionately with body size. Depending on the species, the smallest males either do not produce a horn at all, or they produce a disproportionately small horn for their body size. While the diversity of horn shapes and their behavioural ecology have been reasonably well studied, we know far less about the proximate mechanisms that regulate horn growth. Thus, using 454 pyrosequencing, we generated transcriptome profiles, during horn growth and development, in two different scarab beetle species: the Asian rhinoceros beetle, Trypoxylus dichotomus, and the dung beetle, Onthophagus nigriventris. We obtained over half a million reads for each species that were assembled into over 6,000 and 16,000 contigs respectively. We combined these data with previously published studies to look for signatures of molecular evolution. We found a small subset of genes with horn-biased expression showing evidence for recent positive selection, as is expected with sexual selection on horn size. We also found evidence of relaxed selection present in genes that demonstrated biased expression between horned and horn-less morphs, consistent with the theory of developmental decoupling of phenotypically plastic traits.

The nutritionally responsive transcriptome of the polyphenic beetle Onthophagus taurus and the importance of sexual dimorphism and body region

“…These results build on findings from other studies that show that morph-biased genes subjected to sexual selection (i.e. those in male head horns) show strong signatures of purifying and positive selection and greater evolutionary divergence relative to genes with less morph-bias [51,66]. These results emphasize that sexual selection intensity may interact with relaxed selection associated with environmentbiased gene expression to result in different signatures of selection across traits.…”

Transcriptome analysis reveals the genetic basis underlying the seasonal development of keratinized nuptial spines in Leptobrachium boringii

“…In addition, the mapping rates of clean reads to the assembled unigenes for all samples ranged from 75.70% to 81.83% (Additional file 1: Table S1). Taken together, our de novo assemblies revealed a high quality compared with previous studies [5, 34, 35]. This high quality of sequence reads and assembly was the foundation of all our subsequent analyses [36].…”

“…Thus, SSTs typically evolved in male individuals [1–3]. The best studied SSTs are elaborate ornaments or weapons, such as the peacock’s tail [4], horns of scarab beetles [5], swords of the swordtail fish [6], and antlers of the deer [7]. Among SSTs, weapons have evolved multiple times across the animal kingdom [8].…”

“…Within species, characteristics of male SSTs in general and the size of weapons in particular strongly depend on individual body size, physiological condition, nutritional condition and genetic quality [5, 10–14]. Moreover, the expression of diverse male SSTs varies with a populations’ breeding cycle [15–18].…”

Evolution of allometric changes in fruit fly legs: a developmentally entrenched story

“…In addition to this approach, a few studies have combined these morphological measurements with various genetic manipulations to understand how allometry evolves in more detail (Brakefield, 2006;Carreira et al, 2009;Vasseur et al, 2012;Warren et al, 2014). For example, artificial selection experiments in the butterfly, Bicyclus anynana, show the potential for incredible flexibility in allometry during evolution (Beldade and Brakefield, 2002;Frankino et al, 2005).…”