HETEROSIS: IMPROVEMENT IN ANIMALS, PLANTS, HUMAN - BIOLOGY - 2025

The heterosis, also known as heterozygote advantage and hybrid vigor, is a genetic phenomenon that manifests itself in the improvement with respect to parents, physiological performance in the first generation of the cross between distant relatives of the same species, or between different species, of plants and animals.

The improvement of physiological performance occurs, for example, in the increase of health, cognitive capacity or mass, referring to advantageous phenotypic characteristics resulting from having a more suitable genotype.

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It should be noted that by distant relatives we understand individuals from genetically isolated populations, as well as varieties, strains, or subspecies of the same species.

Inbreeding depression

Heterosis is the result of exogamy. It is the opposite of inbreeding, which can produce homozygosity. Due to genetic recombination, the advantages of heterozygotes may disappear, by reappearance of homozygosity, and even by sterility, in the second generation.

However, the genetic exchange between distant relatives can confer adaptive advantages in the long term.

Inbreeding depression is the reduction of adaptability (fitness) caused by inbreeding. It is expressed as a reduction in survival and reproduction in the progeny of related individuals with respect to the progeny of unrelated individuals. It is a universal phenomenon that has been documented in plants and animals.

When there is a crossing between distant relatives of the same species, or between different species, the result is usually the incorporation of new or rare alleles (introgression) to the gene pool of the population to which the members of the generation resulting from the initial crossing.

In fact, exogamy is often a more important source of new or rare alleles than mutation. These alleles confer two advantages: 1) they increase genetic variability and therefore the frequency of heterozygous individuals in said population; 2) introduce genes that code for phenotypic traits that represent novel preadaptations.

Genetic advantages

From the point of view of Mendelian genetics, the advantages of heterosis have been explained by two hypotheses: 1) complementation, also referred to as the dominance model; 2) allelic interaction, also referred to as the over-dominance model.

The complementation hypothesis postulates that, at multiple genetic loci, heterozygous progeny express fewer slightly deleterious recessive alleles than their homozygous parents.

In hybrid progeny, the higher alleles of one parent would hide the lower alleles of the other parent. This would mean that, for each of the genetic loci involved, the progeny express only the best of the alleles from both parents.

Thus, the first generation would possess a cumulatively fitter genotype with the best characteristics of each parent.

The allelic interaction hypothesis postulates that the two alleles of each genetic loci express complementary, that is, they add their effects. This means that the phenotypic characters encoded by both alleles could produce a broader response to the environmental variability faced by the progeny than is allowed by homozygosity.

These two hypotheses are not mutually exclusive in the sense that each of them could be applied to different sets of genetic loci in the same hybrid individual.

In plants

At the beginning of the 20th century, George Shull showed that hybridizing two varieties of corn grown in the United States, which had lost some of their productivity due to inbreeding, produced larger and more vigorous plants with superior yields. Currently, in hybrid maize, heterosis allows to obtain 100–200% larger harvests.

In the late 1970s, China began cultivating hybrid rice that produced 10% higher yields than conventional maize. Currently, 20–50% larger harvests are achieved

The yield increases achieved by heterosis in other edible cultivated plants are: aubergine, 30–100%; broccoli, 40–90%; zucchini, 10–85%; barley, 10–50%; onion, 15–70%; rye, 180–200%; rapeseed, 39–50%; broad beans, 45–75%; wheat, 5–15%; carrot, 25–30%.

In animals

Mules are the most famous animal hybrid. They result from mating a male horse (Equus caballus) with a female donkey (E. asinus). Their usefulness as pack animals is due to heterosis. They are bigger, stronger and more resistant than the horse. They have the safe step of the ass. They also have a greater capacity for learning than their parents.

The hybridization of macaques (Macaca mulatta) of Chinese and Hindu origin produces males and females that show heterosis due to the fact that they are of greater head-body length and greater body mass than their parents. This difference is more marked in males, which could improve their ability to compete with non-hybrid males for females.

The edible frog (Pelophylax esculentus) is the fertile hybrid of Pelophylax ridibundus and P. lessonae (Ranidae family) that live in sympatry in central Europe. P. esculentus resists lower oxygen pressures than parent species, allowing it to hibernate in severely oxygen-deficient waters. Where they coexist, P. esculentus is more abundant.

In the human being

Currently, our planet is inhabited by a single human species. There is genetic evidence that 65,000–90,000 years ago modern European humans (Homo sapiens) occasionally hybridized with Neanderthals (Homo neanderthalensis).

There is also evidence to indicate that modern Melanesian humans (Homo sapiens) interbred quite frequently with Denisovans, a mysterious extinct human species, 50,000–100,000 years ago.

It is not known whether these ancient hybridizations resulted in heterosis, but it is possible that this is the case based on the observation of positive and negative heterosis in modern humans.

People with fathers and mothers from different parts of China have been shown to have higher heights and academic performance than the averages of their parents' regions of origin. This can be interpreted as positive heterosis.

Many different ethnic groups live in Pakistan characterized by high levels of homozygosity caused by the high frequency of consanguineous marriages. These groups are thought to suffer from negative heterosis, which is expressed in the higher than normal incidence of breast and ovarian cancers.

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