Species barriers, expressed as hybrid inviability and sterility, are often due to epistatic interactions between divergent loci from two lineages. Theoretical models indicate that the strength, direction, and complexity of these genetic interactions can strongly affect the expression of interspecific reproductive isolation and the rates at which new species 5 evolve. Nonetheless, empirical analyses have not quantified the frequency with which loci are involved in interactions affecting hybrid fitness, and whether these loci predominantly interact synergistically or antagonistically, or preferentially involve loci that have strong individual effects on hybrid fitness. We systematically examined the prevalence of interactions between pairs of short chromosomal regions from one species 10 (Solanum habrochaites) co-introgressed into a heterospecific genetic background (Solanum lycopersicum). We used lines containing pairwise combinations of 15 chromosomal segments from S. habrochaites crossed into the background of S. lycopersicum (i.e., 95 double introgression lines). We compared the strength of hybrid incompatibility (either pollen sterility or seed sterility) expressed in each double 15 introgression line to the expected additive effect of its two component single introgressions. We found that: epistasis was common among co-introgressed regions; epistastic effects for hybrid dysfunction were overwhelmingly antagonistic (i.e., double hybrids were less unfit than expected from additive single introgression effects); and, epistasis was substantially more prevalent in pollen fertility compared to seed fertility 20 phenotypes. Together, these results indicate that higher-order interactions frequently contribute to postzygotic sterility barriers in these species. This pervasive epistasis leads to the decoupling of the patterns of accumulation of isolation loci and isolation phenotypes, and is expected to attenuate the rate of accumulation of hybrid infertility among lineages over time (i.e., giving diminishing returns as more reproductive isolation 25 loci accumulate). This decoupling effect might also explain observed differences between pollen and seed fertility in their fit to theoretical predictions of the accumulation of isolation loci, including the 'snowball' effect.. CC-BY-NC-ND 4.0 International license peer-reviewed) is the author/funder. It is made available under aThe copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/090886 doi: bioRxiv preprint first posted online Dec. 1, 2016; AUTHOR SUMMARY 30A characteristic feature of new species is their inability to produce fertile or viable hybrids with other lineages. This post-zygotic reproductive isolation is caused by dysfunctional interactions between genes that have newly evolved changes in the diverging lineages. Whether these interactions occur between pairs of divergent alleles, or involve more complex networks of genes, can have strong effects on how rapidly 35 reproductive isolation-and therefore new species-evolve. The...
The West-Brown-Enquist (WBE) metabolic scaling theory posits that many organismal features scale predictably with body size because of selection to minimize transport costs in resource distribution networks. Many scaling exponents are quarter-powers, as predicted by WBE, but there are also biologically significant deviations that could reflect adaptation to different environments. A central but untested prediction of the WBE model is that wide deviation from optimal scaling is penalized, leading to a pattern of constraint on scaling exponents. Here, we demonstrate, using phylogenetic comparative methods, that variation in allometric scaling between mass and leaf area across 17 wild tomato taxa is constrained around a value indistinguishable from that predicted by WBE but significantly greater than 2/3 (geometric-similarity model). The allometric-scaling exponent was highly correlated with fecundity, water use, and drought response, suggesting that it is functionally significant and therefore could be under selective constraints. However, scaling was not strictly log-log linear but rather declined during ontogeny in all species, as has been observed in many plant species. We caution that although our results supported one prediction of the WBE model, it did not strongly test the model in other important respects. Nevertheless, phylogenetic comparative methods such as those used here are powerful but underutilized tools for metabolic ecology that complement existing methods to adjudicate between models.
Stomata regulate the supply of CO 2 for photosynthesis and the rate of water loss out of the leaf. The presence of stomata on both leaf surfaces, termed amphistomy, increases photosynthetic rate, is common in plants from high light habitats, and rare otherwise. In this study I use optimality models based on leaf energy budget and photosynthetic models to ask why amphistomy is common in high light habitats. I developed an R package leafoptimizer to solve for stomatal traits that optimally balance carbon gain with water loss in a given environment. The model predicts that amphistomy is common in high light because its marginal effect on carbon gain is greater than in the shade, but only if the costs of amphistomy are also lower under high light than in the shade. More generally, covariation between costs and benefits may explain why stomatal and other traits form discrete phenotypic clusters.
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