Evidence for hybrid breakdown in production of red carotenoids in the marine invertebrate Tigriopus californicus

The marine copepod, Tigriopus californicus, produces the red carotenoid pigment astaxanthin from yellow dietary precursors. This ‘bioconversion’ of yellow carotenoids to red is hypothesized to be linked to individual condition, possibly through shared metabolic pathways with mitochondrial oxidative phosphorylation. Experimental inter-population crosses of lab-reared T. californicus typically produces low-fitness hybrids is due in large part to the disruption of coadapted sets nuclear and mitochondrial genes within the parental populations. These hybrid incompatibilities can increase variability in life history traits and energy production among hybrid lines. Here, we tested if production of astaxanthin was compromised in hybrid copepods and if it was linked to mitochondrial metabolism and offspring development. We observed no clear mitonuclear dysfunction in hybrids fed a limited, carotenoid-deficient diet of nutritional yeast. However, when yellow carotenoids were restored to their diet, hybrid lines produced less astaxanthin than parental lines. We observed that lines fed a yeast diet produced less ATP and had slower offspring development compared to lines fed a more complete diet of algae, suggesting the yeast-only diet may have obscured effects of mitonuclear dysfunction. Astaxanthin production was not significantly associated with development among lines fed a yeast diet but was negatively related to development in early generation hybrids fed an algal diet. In lines fed yeast, astaxanthin was negatively related to ATP synthesis, but in lines fed algae, the relationship was reversed. Although the effects of the yeast diet may have obscured evidence of hybrid dysfunction, these results suggest that astaxanthin bioconversion may still be related to mitochondrial performance and reproductive success.

Hybrid Incompatibilities and Transgressive Gene Expression Between Two Closely Related Subspecies of Drosophila

Genome-wide assays of expression between species and their hybrids have identified genes that become either over- or underexpressed relative to the parental species (i.e., transgressive). Transgressive expression in hybrids is of interest because it highlights possible changes in gene regulation linked to hybrid dysfunction. Previous studies in Drosophila that used long-diverged species pairs with complete or nearly complete isolation (i.e., full sterility and partial inviability of hybrids) and high-levels of genome misregulation have found correlations between expression and coding sequence divergence. The work highlighted the possible effects of directional selection driving sequence divergence and transgressive expression. Whether the same is true for taxa at early stages of divergence that have only achieved partial isolation remains untested. Here, we reanalyze previously published genome expression data and available genome sequence reads from a pair of partially isolated subspecies of Drosophila to compare expression and sequence divergence. We find a significant correlation in rates of expression and sequence evolution, but no support for directional selection driving transgressive expression in hybrids. We find that most transgressive genes in hybrids show no differential expression between parental subspecies and used SNP data to explore the role of stabilizing selection through compensatory mutations. We also examine possible misregulation through cascade effects that could be driven by interacting gene networks or co-option of off-target cis-regulatory elements.

Speciation and the developmental alarm clock

New species arise as the genomes of populations diverge. The developmental ‘alarm clock’ of speciation sounds off when sufficient divergence in genetic control of development leads hybrid individuals to infertility or inviability, the world awoken to the dawn of new species with intrinsic post-zygotic reproductive isolation. Some developmental stages will be more prone to hybrid dysfunction due to how molecular evolution interacts with the ontogenetic timing of gene expression. Considering the ontogeny of hybrid incompatibilities provides a profitable connection between ‘evo-devo’ and speciation genetics to better link macroevolutionary pattern, microevolutionary process, and molecular mechanisms. Here, we explore speciation alongside development, emphasizing their mutual dependence on genetic network features, fitness landscapes, and developmental system drift. We assess models for how ontogenetic timing of reproductive isolation can be predictable. Experiments and theory within this synthetic perspective can help identify new rules of speciation as well as rules in the molecular evolution of development.

Speciation and the developmental alarm clock

New species arise as the genomes of populations diverge. The developmental ‘alarm clock’ of speciation sounds off when sufficient divergence in genetic control of development leads hybrid individuals to infertility or inviability, the world awoken to the dawn of new species with intrinsic post-zygotic reproductive isolation. Some developmental stages will be more prone to hybrid dysfunction due to how molecular evolution interacts with the ontogenetic timing of gene expression. Considering the ontogeny of hybrid incompatibilities provides a profitable connection between ‘evo-devo’ and speciation genetics to better link macroevolutionary pattern, microevolutionary process, and molecular mechanisms. Here we explore speciation alongside development, emphasizing their mutual dependence on genetic network features, fitness landscapes, and developmental system drift. We assess models for how ontogenetic timing of reproductive isolation can be predictable. Experiments and theory within this synthetic perspective can help identify new rules of speciation as well as rules in the molecular evolution of development.Impact StatementIntegrating speciation genetics with ontogeny can identify predictable rules in the molecular evolution of developmental pathways and in the accumulation of reproductive isolation as genomes diverge.

Genetic analysis of hybrid incompatibility suggests transposable elements increase reproductive isolation in the D. virilis clade

Although observed in many interspecific crosses, the genetic basis of most hybrid incompatibilities is still unknown. Mismatches between parental genomes in selfish elements and the genes that regulate these elements are frequently hypothesized to underlie hybrid dysfunction. We evaluated the potential role of transposable elements (TEs) in hybrid incompatibilities by examining hybrids between Drosophila virilis strains polymorphic for TEs that cause dysgenesis and a closely related species that appears to lack these elements. Using genomic data, we confirmed copy number differences in potentially causal TEs between the dysgenic-causing D. virilis (TE+) strain and a sensitive D. virilis (TE-) strain and D. lummei genotype. We then contrasted isolation phenotypes in a cross where dysgenic TEs are absent from both D. virilis (TE-) and D. lummei parental genotypes, to a cross where dysgenic TEs are present in the D. virilis (TE+) parent and absent in the D. lummei parent, predicting increased reproductive isolation in the latter cross. Using F1 and backcross experiments that account for alternative hypotheses, we demonstrated amplified reproductive isolation specifically in the interspecific cross involving TE+ D. virilis, consistent with the action of dysgenesis-inducing TEs. These experiments demonstrate that TEs can contribute to hybrid incompatibilities via presence/absence polymorphisms.

Divergent mitochondrial and nuclear OXPHOS genes are candidates for genetic incompatibilities inFicedulaFlycatchers

Hybrid dysfunction is an important source of reproductive isolation between emerging species. Bateson-Dobzhansky-Muller incompatibilities are theoretically well-recognized as the underlying cause of low hybrid dysfunction. However, especially in wild populations, little empirical evidence exists for which genes are involved in such incompatibilities. The relative role of ecological divergence in causing the build-up of genetic incompatibilities in relation to other processes such as genomic conflict therefore remains largely unknown. Genes involved in energy metabolism are potential candidates for genetic incompatibilities, since energy metabolism depends on co-expression of mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) leading to mitonuclear coadaptation. When mitochondrial and nuclear genes lacking a co-evolutionary history appear together in hybrids, incompatibilities could arise.Ficedulaflycatcher F1 hybrids have a higher resting metabolic rate (RMR) compared to the parental species, which could be a sign of genetic incompatibilities between energy metabolism genes that diverged in response to environmental differences while the species were in allopatry. Based on sequences of 15 mitochondrial genes of 264 individuals, we show that the two species have divergent mtDNA caused by the build-up of mainly synonymous mutations and a few non-synonymous mutations. Pied flycatcher mitogenomes show evidence of non-neutrality, indicating a selective sweep or population expansion. There is little variation in the nuclear OXPHOS-related proteins and no significant deviation from neutrality, however, specific codon identified sites might be under positive selection in both mitochondrial and nuclear genes encoding OXPHOS proteins for complex I and III. Taken together, these diverged mitonuclear genes therefore constitute possible candidates underlying, at least part of the genetic incompatibilities that cause hybrid dysfunction in crosses between collared and pied flycatchers.

Molecular evolution across developmental time reveals rapid divergence in early embryogenesis

Ontogenetic development hinges on the changes in gene expression in time and space within an organism, suggesting that the demands of ontogenetic growth can impose or reveal predictable pattern in the molecular evolution of genes expressed dynamically across development. Here we characterize co-expression modules of the C. elegans transcriptome, using a time series of 30 points from early-embryo to adult. By capturing the functional form of expression profiles with quantitative metrics, we find fastest evolution in the distinctive set of genes with transcript abundance that declines through development from a peak in young embryos. These genes are highly enriched for oogenic function (maternal provisioning), are non-randomly distributed in the genome, and correspond to a life stage especially prone to inviability in inter-species hybrids. These observations conflict with the “early conservation model” for the evolution of development, though expression-weighted sequence divergence analysis provides some support for the “hourglass model.” Genes in co-expression modules that peak toward adulthood also evolve fast, being hyper-enriched for roles in spermatogenesis, implicating a history of sexual selection and relaxation of selection on sperm as key factors driving rapid change to ontogenetically distinguishable co-expression modules of genes. We propose that these predictable trends of molecular evolution for dynamically-expressed genes across ontogeny predispose particular life stages, early embryogenesis in particular, to hybrid dysfunction in the speciation process.Impact SummaryThe development of an organism from a single-celled embryo to a reproductive adult depends on dynamic gene expression over developmental time, with natural selection capable of shaping the molecular evolution of those differentially-expressed genes in distinct ways. We quantitatively analyzed the dynamic transcriptome profiles across 30 timepoints in development for the nematode C. elegans. In addition to rapid evolution of adult-expressed genes with functional roles in sperm, we uncovered the unexpected result that the distinctive set of genes that evolve fastest are those with peak expression in young embryos, conflicting with some models of the evolution of development. The rapid molecular evolution of genes in early embryogenesis contrasts with the exceptional conservation of embryonic cell lineages between species, and corresponds to a developmental period that is especially sensitive to inviability in inter-species hybrid embryos. We propose that these predictable trends of molecular evolution for dynamically-expressed genes across development predispose particular life stages, early embryogenesis in particular, to hybrid dysfunction in the speciation process.