The genetic origins behind reproductive traits are still far from clear, and this is especially so for horses, researchers say. Photo by Phinehas Adams

The knowledge of the reproductive architecture of horses on a genetic level is likely to grow considerably over the next few years, the authors of a just-published review predict.

The genetic origins behind reproductive traits are still far from clear, and this is especially so for horses.

Nora Laseca and her colleagues, writing in the journal Animals, said reproductive traits are complex and usually determined by multiple genes.

They also have low heritability, which makes them particularly sensitive to environmental and management factors, such as age, nutrition, training, temperature at mating, and breeding season.

By that reason, modeling reproductive traits from a genetic point of view is difficult, they wrote in their review of recent advances and studies analyzing genomic mechanisms affecting reproductive function in mares and stallions.

This is particularly important in equines, whose fertility is considerably lower than that observed in other domestic species, they said.

In addition, equine reproductive efficiency is limited by their own physiology, characterized by single births in almost all the foalings, seasonality in mares, long generation intervals delaying genetic improvement, and a lack of systematized collection of phenotypic information reproductive traits.

Nevertheless, a certain degree of genetic influence has been described and modelized from a quantitative viewpoint, in mares and stallions.

For instance, gestation length in mares is affected by maternal lineage or inbreeding, while changes in sperm traits are related to the genetic background of the individual, lineage and breed, as well as by the inbreeding value.

However, there are very few reports which describe the molecular mechanisms involved in such genetic influence or which detect the candidate genes involved in the biological processes.

The advent of genomics has led to the development of new approaches to genetic analysis in livestock, including the detection of specific mutations and other genetic issues.

However, their use in equines is still limited, probably due to the delay in the development of a reliable reference genome, in comparison with most livestock species, such as pigs or cattle, but also by a lack of reliable expected progeny differences and phenotypic values associated to the variations in fertility in the species.

Laseca and her colleagues noted that, although the use of genomic methods in horses has increased significantly over the past five years, studies focusing on reproductive traits remain few and far between.

There are even fewer which aim to dissect and quantify more accurately the influence of the genetic background and the environment in the expression of the observable traits, they noted.

Horse genomics is currently undergoing an exponential expansion, not least due to the adaptation of new genomic methodologies to the species, the existence of a new, accurate reference genome, and the exponential increase in the number of equines which have been genotyped.

It is, therefore, highly likely that our knowledge of the reproductive architecture of horses will grow considerably over the next few years.

However, large-scale datasets of reproductive phenotypes are still scarce in horses, probably due to the lack of availability of reliable reproductive phenotypes (particularly in mares).

Therefore, the development of new phenotypes to measure reproductive fitness more objectively and their systematic use by breeder associations are essential to allow a more in-depth study of the reproductive function in horses.

In addition, the search for candidate genes is a highly promising way to obtain a better understanding of the processes involved in horse fertility.

They not only can help to elucidate which physiological functions could be affected by a specific genotype, but also to predict which genotypes could be more affected by environmental challenges, and they can be integrated into breeding programs to detect, even at very young ages, the potential fertility (increased, normal, or decreased) of a given mare or stallion.

The authors said the lack of consistency observed in candidate genes associated with fertility detected in different breeds or populations of horses, (most of them detected by a single study) will be reduced only with an increase in the volume of evidence gathered.

In this sense, only 11 genes reviewed in their paper were detected by two or more studies, and only five were detected using more than one approach.

It was recently shown that combined genomic approaches in the same study can increase accuracy and reliability in detecting candidate genes. However, no combined studies have yet been reported in horses. Such combined approaches would constitute the best approach in our search for a better, more reliable understanding of genetic effects on horse fertility.

The review team comprised Laseca, Gabriel Anaya, Zahira Pea and Antonio Molina, with the University of Crdoba in Spain; and Yamila Pirosanto and Sebastin Demyda Peyrs, with the National University of La Plata and the Higher Council for Scientific and Technological Research, both in La Plata, Argentina.

Laseca, N.; Anaya, G.; Pea, Z.; Pirosanto, Y.; Molina, A.; Demyda Peyrs, S. Impaired Reproductive Function in Equines: From Genetics to Genomics. Animals 2021, 11, 393. https://doi.org/10.3390/ani11020393

The review, published under a Creative Commons License, can be read here.

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Window into the genetics behind equine reproduction grows ever wider, say researchers - Horsetalk