Heterosis is the phenomenon whereby first generation (F1) hybrids have higher trait values than their parents (Charlesworth and Willis 2009). These F1 hybrids can be produced by parents of different species that are able to hybridize, or of different populations of the same species. Fitness measures like height, weight, stress tolerance, metabolic efficiency and egg production are increased above mid-parent value or even above the best performing parent. However, true Darwinian fitness is not increased due to infertility of the F1 hybrids or due to developmental defects in the second hybrid generation (F2) offspring. Heterosis has been found in all vertebrate classes, plants, insects and nematodes, making it a widespread phenomenon. It is therefore a very interesting biological phenomenon. It is also employed in agriculture to increase the yield of many crops. Conservation biology could also benefit from knowledge on heterosis to enrich the genetic composition of small populations. However, the causes for heterosis are not fully understood. Three questions are proposed to elucidate the genetic basis of heterosis and to explain the patterns found in heterosis.
What is the genetic basis for heterosis?
Three main theories have been proposed to answer this question: overdominance theory, partial dominance theory and epistatic interaction theory.
Overdominance occurs when the heterozygote on a locus has superior fitness compared to both homozygotes. Small populations or even species can be fixated on one of the alleles, making heterozygotes absent. When hybridization between the two populations or species occurs, heterozygosity is restored and fitness is increase due to the overdominant loci.
Partial dominance theory states that deleterious mutations that are found in a population are more often recessive compared to the wildtype allele. Populations or species can have multiple deleterious alleles fixated, decreasing their fitness. Because populations tend to fixate different recessive deleterious alleles, F1 hybrids can be ‘rescued’ from the deleterious effects of the recessive mutations that are contributed by one parent by the dominant wildtype allele contributed from the other parent.
Epistatic interaction theory states that genes with epistatic interactions in species or populations can diversify compared to the common ancestor. When the F1 hybrid is formed, new, superior, epistatic interactions are established between genes of the different populations. Therefore the F1 hybrid has superior performance over both parents.
Why is reproduction negatively influenced in F1 hybrids?
Many F1 hybrids males are sterile, especially when separate species are the parents, and also the fertility or fecundity of F1 hybrid females is reduced. The explanation for the difference in fertility of males and females can be sought in Haldane’s rule, which states that the heterogametic sex suffers most from hybridization. The dominance theory of Haldane’s rule is the most accepted theory, and states that the X or W chromosome from one parental species is not accompanied by a homologue of the other parent, causing interactions between autosomes of the other species and the X or W of the other species to lose function or effectiveness.
Why are F2 offspring often inferior?
F2 offspring are often inferior compared to the F1 generation, and when separated species are involved also often to the parental individuals. This is caused by the recombination that took place during F1 meiosis. Every chromosome will carry parts from either parent, making a fusion between two gametes of F1 hybrids produce zygotes that are homozygous for the alleles contributed by one parent. Because some alleles or even genes of one parent are lost, only flawless interaction between the genomes can produce fit offspring. Because flawless interaction between gene complexes that are not co-adapted is extremely rare, F2 offspring will have reduced fitness.