Patterns of Transcriptome Divergence in the Male Accessory Gland of Two Closely Related Species of Field Crickets

Abstract

One of the central questions in evolutionary genetics is how much of the genome is involved in the early stages of divergence between populations, causing them to be reproductively isolated. In this article, we investigate genomic differentiation in a pair of closely related field crickets (Gryllus firmus and G. pennsylvanicus). These two species are the result of allopatric divergence and now interact along an extensive hybrid zone in eastern North America. Genes encoding seminal fluid proteins (SFPs) are often divergent between species, and it has been hypothesized that these proteins may play a key role in the origin and maintenance of reproductive isolation between diverging lineages. Hence, we chose to scan the accessory gland transcriptome to enable direct comparisons of differentiation for genes known to encode SFPs with differentiation in a much larger set of genes expressed in the same tissue. We have characterized differences in allele frequency between two populations for .6000 SNPs and .26,000 contigs. About 10% of all SNPs showed nearly fixed differences between the two species. Genes encoding SFPs did not have significantly elevated numbers of fixed SNPs per contig, nor did they seem to show larger differences than expected in their average allele frequencies. The distribution of allele frequency differences across the transcriptome is distinctly bimodal, but the relatively high proportion of fixed SNPs does not necessarily imply "ancient" divergence between these two lineages. Further studies of linkage disequilibrium and introgression across the hybrid zone are needed to direct our attention to those genome regions that are important for reproductive isolation.T HE study of speciation, defined as the origin of intrinsic barriers to gene exchange (Mayr 1942;Harrison 1998;Coyne and Orr 2004), relies on comparisons of phenotypes and genotypes among diverging populations, strains, subspecies, or closely related species. In recently diverged taxa, observed differences in genotypes or phenotypes are likely to be associated with the origin of reproductive barriers and less likely to be differences that have accumulated subsequent to initial divergence. As Templeton (1981) emphasized, our ultimate goal is to understand the genetics of speciation, not simply the genetics of species differences.It is now widely recognized that the amount of divergence between populations or species will vary across the genome due to selective and random lineage sorting from polymorphic ancestral populations and differential introgression when diverging taxa hybridize where their distributions overlap (Harrison 1991;Wu 2001;Turner et al. 2005;Nosil et al. 2009). Barton and Hewitt (1981) explicitly argued that gene exchange between hybridizing taxa will depend on genome region. Differential introgression has been widely discussed and documented in the hybrid zone literature (Harrison 1990;Rieseberg et al. 1999;Payseur 2010), where species boundaries have been described as semipermeable. Chromosome regions that harbo...

Population Genetics and a Study of Speciation Using Next-Generation Sequencing: An Educational Primer for Use with “Patterns of Transcriptome Divergence in the Male Accessory Gland of Two Closely Related Species of Field Crickets”

“…Based on these observations, it has been proposed that divergent seminal fluid proteins might contribute to the early stages of speciation in closely related species of field crickets Andrés et al (2006). Andrés et al (2013) explore this hypothesis by characterizing DNA sequence variation in the coding regions of proteins expressed in accessory glands where seminal proteins are found.…”

“…The senior author of the Andrés et al (2013) study, Richard G. Harrison, began investigating speciation in crickets in the mid-1970s using protein electrophoresis techniques. Since then, he and his colleagues have examined properties of cricket mitochondrial DNA Harrison 1986, 1989), studied hybrid zones where different species of crickets meet (Willett et al 1997;Ross and Harrison 2002;Maroja et al 2009), and investigated whether the bacterial parasite Wolbachia (Mandel et al 2001;Maroja et al 2008) and divergent reproductive proteins (Andrés et al 2006(Andrés et al , 2008 contribute to speciation in crickets.…”

“…Gene flow through a hybrid zone accentuates regions of the genome contributing to reproductive isolation by decreasing differences in DNA sequence elsewhere in the genome. Based on the idea that divergent seminal fluid proteins have played an important role in speciation, Andrés et al (2013) hypothesized that they would see greater sequence divergence between G. firmus and G. pennsylvanicus in genes encoding seminal fluid proteins than in genes encoding other proteins expressed in the same tissue (the accessory gland).…”

“…These differences in gene expression make one cell type different from another. Because Andrés et al (2013) are interested in the evolution of genes that can disrupt interactions between sperm and eggs, they examined genes expressed in the male accessory gland. That is, they sequenced the accessory gland transcriptome, the subset of sequences in the genome that are transcribed to produce RNA in accessory glands.…”

“…To isolate DNA sequences coding for proteins found in the accessory gland, Andrés et al (2013) dissected accessory glands from G. firmus and G. pennsylvanicus males, extracted RNA from each tissue sample, and used reverse transcriptase to synthesize DNA complementary to the RNA (termed "cDNA"). Consequently, the cDNA samples contained only DNA sequences from genes that were transcribed into RNA.…”