Above: A liger (a hybird cross between a male lion and a tigress) at the Novosibirsk Zoo in Russia (Wikimedia Commons/Aleksej Shilin)
What is hybrid vigour?
A hybrid is something that combines two or more different things. For example, a hybrid car is a vehicle that can be powered by different sources of fuel, such as gasoline and electricity.
Did you know? Charles Darwin was one of the first people to describe hybrid vigour. He found that the offspring of plants called Common Toadflax were larger when the parents came from different strains.
Hybrid organisms are living things whose parents are genetically very different. The hybrid’s parents may be from different breeds, or different subspecies within the same species. Or they may even be from two entirely different species.
Hybrid vigour (also called heterosis) describes cases where the hybrid offspring exhibits qualities that are more desirable than those of either of its parents. For example, the hybrid might be larger, more disease-resistant, physically stronger, or capable of producing more offspring than either parent.
Homozygosity and heterozygosity
In sexually reproducing organisms, an individual receives one set of genes from each parent. For example, an animal normally receives one set of genes from its mother, and one from its father.
In any population, there are usually many different versions of each gene available. These versions are called alleles. When both copies of a gene are identical—when both parents contribute the same allele—the offspring is homozygous for that gene. When two different alleles are paired, the offspring is heterozygous for that gene.
Since hybrids have two very different parents, they are almost always heterozygous.
Hybrid advantages (and drawbacks)
Did you know? North American farmers normally detassle their corn (remove the flower) to force crosses between different varieties grown in the same field. This allows for high-quality first-generation hybrids year after year.
It is usually better to be heterozygous than homozygous. For example, heterozygosity is associated with improved resistance to disease. This is particularly true in the case of major histocompatibility complex (MHC) genes, which help determine the immune system’s ability to recognize foreign substances. The more different types of MHC genes you have, the broader the range of pathogens (germs) your immune system can recognize.
Heterozygous plants and animals are also less likely to express deleterious (harmful) mutations or alleles, because they will usually have at least one copy of a gene still intact. They can therefore make up for the fact that the other allele is “broken.”
Sometimes, having one “broken” allele and one good allele is actually even better than having two good alleles! This is called heterozygote advantage. For example, carriers of the Sickle-Cell Anemia gene have only one allele for the disease and don’t develop symptoms. Sickle-cell carriers are also resistant to the blood-borne parasite that causes malaria.
However, hybrids are also susceptible to complex genetic mechanisms like epistasis, where the role played one gene is affected by the presence of other genes. Indeed, hybrids sometimes turn out to have less desirable traits that either of their parents. This is called negative heterosis.
It is generally not a good idea to breed hybrids with each other to create second-generation (F2) hybrids, since first-generation (F1) hybrids share the same gene pool. That makes their offspring even more likely to have some homozygous genes. The traits exhibited by F2 hybrids are usually inferior to F1s.
Hybrid vigour and agriculture
Did you know? Examples of animals that show hybrid vigour include wolf-dogs (crosses between wolves and domestic dogs), ligers (crosses between male lions and female tigresses), and mules (crosses between female horses and male donkeys).
Throughout human history, farmers have often benefited from hybrid vigour. For example, livestock can be hybridized to produce larger animals for slaughter. Mules (F1 hybrids of horses and donkeys) have traditionally been favoured for farm labour due to their hardiness and strength.
Hybrid plants are also very important in agriculture. For example, different kinds of corn or rice can be hybridized to increase disease resistance and yields. The first-generation hybrids (F1) show the best level of hybrid vigour.
Although they are similar, it is important to remember that hybrid vigour and heterozygote advantage are not the same thing. Hybrid vigour refers to any advantage an organism gains from being the offspring of parents that are genetically very different. Heterozygote advantage is much more specific, referring to an advantage gained from having one “good” copy and one “bad” copy of a specific gene.
In any case, both concepts offer a fascinating perspective on reproduction and evolution.