Population Genetics Breaking Down Hybrids

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POPuLAtION GENEtICS Breaking down hybrids

A general explanation has been hybrids were generated by crossing fitness optimum. From the F2 genera- found for the puzzling perform- Antirrhinum species that have diver- tion onwards, however, this advantage

ance of F1 hybrids, which can have gent CYC or RAD expression levels, breaks down owing to Mendelian increased vigour but also reduced lower expressing hybrids yielded segregation. This model applies to fertility compared with their parents. more ventralized flowers (the figure genes involved in conserved traits, A study in snapdragons suggests that shows a fully ventralized flower). such as physiology or growth, and so these opposing features can arise A visualization tool similar to accounts for the hybrid vigour seen in

from the interaction of genome-wide the GEM map was used to model F1 but not subsequent generations. As expression noise that is phenotypi- how this cryptic expression variation mentioned above, however, the shape cally silent in the parent species. relates to variation in fitness between of the neutral zone depends on the Variation in gene expression parental and hybrid strains. In this particular locus: for genes that have across species is widespread, but link- ‘gene expression–fitness’ map, there is more complex interactions, such as ing this variation to its phenotypic a zone surrounding the fitness opti- those involved in fertility, the shape

consequences requires a trait that is mum, or peak, in which variation in of the neutral zone may make the F1 well characterized genetically and gene expression values is largely neu- generation fall outside the neutral morphologically — one example of tral. The extent of this neutral zone, zone. This scenario might therefore this is the dorsoventral asymmetry and its shape, will depend on the size contribute to the sterility of hybrids, of the Antirrhinum flower. The authors of the population and the selective for example. examined expression variation in two forces acting on the loci. The drifting Existing models to account for the of four transcription-factor genes away of one gene from the optimum properties of hybrids — such as het- that control this conserved trait. would be of little fitness consequence erozygote advantage — have several They then correlated the variation to the population, but the drift of, say, weaknesses, so a statistical explana- in the expression of these two genes 1,000 genes would impose substantial tion based on genotype–fitness maps (CYCLOIDEA (CYC) and RADIALIS fitness costs. for multiple loci has broad appeal (RAD)) with the degree of flower What then is the explanation for and can be tested in other systems.

asymmetry in F1 hybrids. hybrid superiority and inferiority? Tanita Casci First, the phenotypes and expres- Two species will occupy different ORIGINAL RESEARCH PAPER Rosas, U. et al. sion levels of Antirrhinum majus spaces around the fitness peak. When Cryptic variation between species and the basis CYC and RAD mutants were used to those two species are crossed the of hybrid performance. PLoS Biol. 8, e1000429 generate a GEM (gene expression– amount of expression noise of each (2010) morphology) map. Here, genotypes species is partly cancelled out, as the are quantitatively related, through variance in gene expression in their expression levels, to variation in the hybrid is lower than in the flower ventralization. When wild-type parents. This cancelling out brings RAD and CYC alleles of various other the fitness of the hybrid closer to the Antirrhinum species were characterized for their expression levels and positioned on the GEM, they mapped to a plateau, which means that the variation in gene expression that exists within each species did not affect the phenotype. However, crosses between species would be expected to tip the genotypes off this plateau: indeed, when double-heterozygote Image is reproduced from Rosas, U. et al.