Microsatellites as Agents of Adaptive Change: An RNA-Seq-Based Comparative Study of Transcriptomes from Five Helianthus Species

Mutations that provide environment-dependent selective advantages drive adaptive divergence among species. Many phenotypic differences among related species are more likely to result from gene expression divergence rather than from non-synonymous mutations. In this regard, cis-regulatory mutations play an important part in generating functionally significant variation. Some proposed mechanisms that explore the role of cis-regulatory mutations in gene expression divergence involve microsatellites. Microsatellites exhibit high mutation rates achieved through symmetric or asymmetric mutation processes and are abundant in both coding and non-coding regions in positions that could influence gene function and products. Here we tested the hypothesis that microsatellites contribute to gene expression divergence among species with 50 individuals from five closely related Helianthus species using an RNA-seq approach. Differential expression analyses of the transcriptomes revealed that genes containing microsatellites in non-coding regions (UTRs and introns) are more likely to be differentially expressed among species when compared to genes with microsatellites in the coding regions and transcripts lacking microsatellites. We detected a greater proportion of shared microsatellites in 5′UTRs and coding regions compared to 3′UTRs and non-coding transcripts among Helianthus spp. Furthermore, allele frequency differences measured by pairwise FST at single nucleotide polymorphisms (SNPs), indicate greater genetic divergence in transcripts containing microsatellites compared to those lacking microsatellites. A gene ontology (GO) analysis revealed that microsatellite-containing differentially expressed genes are significantly enriched for GO terms associated with regulation of transcription and transcription factor activity. Collectively, our study provides compelling evidence to support the role of microsatellites in gene expression divergence.

Microsatellites as Agents of Adaptive Change: An RNA-seq based Comparative Study of Transcriptomes from Five Helianthus Species

Mutations that provide environment dependent selective advantages drive adaptive divergence among species. Many phenotypic differences among related species are more likely to result from gene expression divergence rather than from non-synonymous mutations. In this regard, cis-regulatory mutations play an important part in generating functionally significant variation. Some proposed mechanisms that explore the role of cis-regulatory mutations in gene expression divergence involve microsatellites. Microsatellites exhibit high mutation rates and are abundant in both coding and non-coding regions and could influence gene function and products. Here we tested the hypothesis that microsatellites contribute to gene expression divergence among species with 50 individuals from nine closely related Helianthus species using an RNA-seq approach. Differential expression analyses of the transcriptomes revealed that genes containing microsatellites in non-coding regions (UTRs and introns) are more likely to be differentially expressed among species when compared to genes with microsatellites in the coding regions and transcripts lacking microsatellites. We detected a greater proportion of shared microsatellites in 5’UTRs and coding regions compared to 3’UTRs and non-coding transcripts among Helianthus spp. Further, allele frequency differences measured by pairwise FST at single nucleotide polymorphisms (SNPs), indicate greater genetic divergence in transcripts containing microsatellites compared to those lacking microsatellites. A gene ontology (GO) analysis revealed that microsatellite-containing differentially expressed genes are significantly enriched for GO terms associated with regulation of transcription and transcription factor activity. Collectively, our study provides compelling evidence to support the role of microsatellites in gene expression divergence.

Experimental sexual selection reveals rapid divergence in male and female reproductive transcriptomes and their interactions

Mating causes substantial changes in females, altering male and female reproductive fitness. Some postmating effects are hypothesized to be at least partially mediated by gene expression changes, driven by postcopulatory sexual selection, which results in population divergence of reproductive proteins that could generate reproductive isolation. However, understanding of the direct role of sexual selection on gene expression divergence along with the subsequent molecular mismatches that could occur between diverging populations is limited. Here, we analyze gene expression divergence following over 150 generations of experimental evolution in which Drosophila pseudoobscura evolved under either elevated polyandry or enforced monogamy. We find that sexual selection rapidly impacted sex-, tissue-, and mating-specific responses, and not always in the predicted direction. Postmating female responses are either unique to each sexual selection treatment or exhibit asymmetric non-congruence, in which monogamy females upregulate and polyandry females downregulate the same genes following mating. This substantial population divergence of gene expression also gives rise to either unique or mismatched gene expression patterns in crosses between treatments. Many of these genes are involved in immune and stress responses, and non-congruent responses are particularly prevalent in the female reproductive tract, the main arena for postcopulatory sexual selection. In summary, we show that sexual selection has pervasive impacts on gene expression divergence acting both differentially between reproductive tissues of the same sex and asymmetrically in postmating female responses, and this divergence is highest in the female reproductive tract, the main arena for postcopulatory sexual selection.

Support for the dominance theory in Drosophila transcriptomes

ABSTRACTInteractions among divergent elements of transcriptional networks from different species can lead to misexpression in hybrids through regulatory incompatibilities, some with the potential to generate sterility. Genes with male-biased expression tend to be overrepresented among genes misexpressed in hybrid males. While the possible contribution of faster-male evolution to this misexpression has been explored, the role of the hemizygous X chromosome (i.e., the dominance theory for transcriptomes) remains yet to be determined. Here we study genome-wide patterns of gene expression in females and males of Drosophila yakuba and D. santomea and their hybrids. We used attached-X stocks to specifically test the dominance theory, and we uncovered a significant contribution of recessive alleles on the X chromosome to hybrid misexpression. Our analysis of gene expression patterns suggests that there is a contribution of weakly deleterious regulatory mutations to gene expression divergence in the sex towards which the expression is biased. In the opposite sex (e.g., genes with female-biased expression analyzed in male transcriptomes), we detect stronger selective constraints on gene expression divergence. Although genes with high degree of male-biased expression show a clear signal of faster-X evolution for gene expression divergence, we also detected slower-X evolution of gene expression in other gene classes (e.g. female-biased genes) that is mediated by significant decreases of cis- and trans-regulatory divergence. The distinct behavior of X-linked genes with high degree of male-biased expression is consistent with these genes experiencing a higher incidence of positively selected regulatory mutations than their autosomal counterparts. We propose that both dominance theory and faster-X evolution of gene expression may be major contributors to hybrid misexpression and possibly the large X-effect in these species.