De novo characterization of the gene-rich transcriptomes of two color-polymorphic spiders, Theridion grallator and T. californicum (Araneae: Theridiidae), with special reference to pigment genes

Abstract

BackgroundA number of spider species within the family Theridiidae exhibit a dramatic abdominal (opisthosomal) color polymorphism. The polymorphism is inherited in a broadly Mendelian fashion and in some species consists of dozens of discrete morphs that are convergent across taxa and populations. Few genomic resources exist for spiders. Here, as a first necessary step towards identifying the genetic basis for this trait we present the near complete transcriptomes of two species: the Hawaiian happy-face spider Theridion grallator and Theridion californicum. We mined the gene complement for pigment-pathway genes and examined differential expression (DE) between morphs that are unpatterned (plain yellow) and patterned (yellow with superimposed patches of red, white or very dark brown).ResultsBy deep sequencing both RNA-seq and normalized cDNA libraries from pooled specimens of each species we were able to assemble a comprehensive gene set for both species that we estimate to be 98-99% complete. It is likely that these species express more than 20,000 protein-coding genes, perhaps 4.5% (ca. 870) of which might be unique to spiders. Mining for pigment-associated Drosophila melanogaster genes indicated the presence of all ommochrome pathway genes and most pteridine pathway genes and DE analyses further indicate a possible role for the pteridine pathway in theridiid color patterning.ConclusionsBased upon our estimates, T. grallator and T. californicum express a large inventory of protein-coding genes. Our comprehensive assembly illustrates the continuing value of sequencing normalized cDNA libraries in addition to RNA-seq in order to generate a reference transcriptome for non-model species. The identification of pteridine-related genes and their possible involvement in color patterning is a novel finding in spiders and one that suggests a biochemical link between guanine deposits and the pigments exhibited by these species.

Positional cloning of a Bombyx pink-eyed white egg locus reveals the major role of cardinal in ommochrome synthesis

“…Although ommochrome pigments are absent in white Bombyx wings, they are present in wings of Nympalidae butterflies such as Heliconius species and Precis coenia (Butenandt and Schafer, 1962;Gilbert et al, 1988;Nijhout, 1997), and phenoxazinone synthetase activity has been detected in pupal P. coenia wings (Nijhout, 1997). We confirmed the presence of cardinal gene homolog in the developing wing expressed sequence tag data set of Heliconius erato (IC33431AfEcon6544) (Hines et al, 2012) by tBlastn search (http://butterflygenome.org/), and the cardinal gene homolog was also reported in the transcriptomes of Hawaiian happy-face spiders (Croucher et al, 2013). It may be of interest to determine whether the cardinal gene ortholog is involved in coloration of butterfly wings and spider bodies.…”

Spiders have melanin after all

“…That the biosynthetic pathway of melanin in spiders was not detected in a recent genomic study may simply illustrate a limitation of homologous sequence comparison to identify evolutionary shifts in enzymes. Genes associated with melanin pigmentation that were not found in spiders include: ebony, eiger, grim, Gustatory receptor 28b, Melanization protein 1, Peptidoglycan recognition protein LC, Serine protease 7, Serpin 27A, Serpin 77Ba, yellow, yellow-f and yellow-f2 (Croucher et al, 2013). However, seven genes associated with melanin synthesis were found in the same study (Croucher et al, 2013): basket, dorsal, hemipterous, Hemolectin, Neuroglian, Rho1 and Toll.…”

“…Genes associated with melanin pigmentation that were not found in spiders include: ebony, eiger, grim, Gustatory receptor 28b, Melanization protein 1, Peptidoglycan recognition protein LC, Serine protease 7, Serpin 27A, Serpin 77Ba, yellow, yellow-f and yellow-f2 (Croucher et al, 2013). However, seven genes associated with melanin synthesis were found in the same study (Croucher et al, 2013): basket, dorsal, hemipterous, Hemolectin, Neuroglian, Rho1 and Toll. Spiders may use different enzymes or substrates to synthesize melanins, as is true in insects that produce pheomelanin (Galván et al, 2015).…”

“…However, pheomelanins have only been very recently reported in invertebrates (Galván et al, 2015;Speiser et al, 2014) and Raman spectrometry may facilitate the discovery of pheomelanins across major lineages of organisms in the future. Despite the ubiquity of melanin in nature, all studies to date have failed to identify melanins in spiders, and a recent genomic study suggested that spiders lack the metabolic pathway known to synthesize melanins endogenously in other organisms (Croucher et al, 2013). Hence, spiders are argued to lack melanins (Foelix, 2011;Holl, 1987;Oxford and Gillespie, 1998;Seligy, 1972) and some chemical data suggest that ommochromes, especially ommins, instead produce their dark colours (Seligy, 1972).…”

Spiders have rich pigmentary and structural colour palettes

“…S1) than 3OH-kynurenine and cannot be detected by Raman, but have been successfully detected by SERS (Smyth et al, 2011). Pterins are abundant in insects (Shamim et al, 2014), and a previous genomic study suggested that pterins could be synthesized de novo in spiders (Croucher et al, 2013). Therefore, pterins could also be responsible for the colour production of these yellow setae.…”