Hybrid dysfunction and physiological compensation in gene expression. Tigriopus californicus

The formation of new species is often a consequence of genetic incompatibilities accumulated between populations during allopatric divergence. When divergent taxa interbreed, these incompatibilities impact physiology and have a direct cost resulting in reduced hybrid fitness. Recent surveys of gene regulation in interspecific hybrids have revealed anomalous expression across large proportions of the genome, with 30-70% of all genes apparently misexpressed, mostly in the direction of down-regulation. However, since most of these studies have focused on pairs of species exhibiting high degrees of reproductive isolation, the association between regulatory disruption and reduced hybrid fitness prior to species formation remains unclear. Within the copepod species Tigriopus californicus, interpopulation hybrids show reduced fitness associated with mitochondrial dysfunction. Here we show that in contrast to studies of interspecific hybrids, only 1.2% of the transcriptome was misexpressed in interpopulation hybrids of T. californicus, and nearly 80% of misexpressed genes were overexpressed rather than underexpressed. Moreover, many of the misexpressed genes were components of functional pathways impacted by mitonuclear incompatibilities in hybrid T. californicus (e.g., oxidative phosphorylation and antioxidant response). We also show that the magnitude of hybrid misregulation is not dependent on levels of protein sequence divergence, even though the latter is correlated with expression divergence between parental populations. Our results suggest that hybrid breakdown at early stages of speciation may result from initial incompatibilities amplified by the cost of compensatory physiological responses. Overall design: Our experiment included nine RNA-seq samples: 3 San Diego, 2 Santa Cruz, and 4 hybrid samples. For each sample, 400-500 copepods across all developmental stages were collected from their stock cultures. They were transferred to fresh filtered seawater in a 50-mL Falcon tubes and immersed in a 20°C water bath for two hours. Water was then quickly removed, 4 mL of Tri-Reagent (Sigma) added, and tissue immediately disrupted using a tissue homogenizer. RNA was isolated following the manufacturer’s protocol. Re-suspended RNA pellets were further purified with RNeasy Mini columns (Qiagen), and final sample integrity and quantity were assessed with an Agilent 2100 BioAnalyzer. Please note that two samples (GSM1531288, GSM1531290) have been accessioned under BioProject PRJNA168170, SRA study SRP013608, while the remaining seven samples under BioProject PRJNA263967, SRA Study SRP048974. The current records including all 9 samples (PRJNA264820/SRP049247) were re-created for the convenient retrieval of the complete raw data from SRA