An issue that has arisen more than once in discussions as of late, specially in relation with the recently discovered Neanderthal admixture in Eurasian Homo sapiens, is that of hybrid vigor or lack of it thereof. This is addressed at a new paper in PLoS Biology:
Ulises Rosas et al., Cryptic Variation between Species and the Basis of Hybrid Performance. PLoS Biology 2010. Open access.
A major conundrum in biology is why hybrids between species display two opposing features. On the one hand, hybrids are often more vigorous or productive than their parents, a phenomenon called hybrid vigor or hybrid superiority. On the other hand they often show reduced vigour and fertility, known as hybrid inferiority. Various theories have been proposed to account for these two aspects of hybrid performance, yet we still lack a coherent account of how these conflicting characteristics arise. To address this issue, we looked at the role that variation in gene expression between parental species may play. By measuring this variation and its effect on phenotype, we show that expression for specific genes may be free to vary during evolution within particular bounds. Although such variation may have little phenotypic effect when each locus is considered individually, the collective effect of variation across multiple genes may become highly significant. Using arguments from theoretical population genetics we show how these effects might lead to both hybrid superiority and inferiority, providing fresh insights into the age-old problem of hybrid performance.
A news article synthesizing the findings can be found at Science Daily:
The results show that hybrids might be expected to exhibit increased performance in basic traits such as growth. However, they also show that in the longer term, other traits such as those involved in sexual reproduction might be expected to perform less well, accounting for reduced fertility of hybrids.