Exploration of the hereditary cause of sex ratio distortion in horses
Sammanfattning: Even sex ratios are seen as evolutionarily stable and are maintained by a selection against a skewed sex ratio. However, sometimes sex ratios at birth are skewed towards one sex. Reasons for this phenomenon are often unknown. The aim of this project was to explore distorted sex ratios in Icelandic horses, Standardbred trotters, and Coldblooded trotters, to estimate the heritability of distorted sex ratios in Icelandic horses and explore families with distorted sex ratios, and set the outline for future research. To investigate the aims, datasets were extracted from the Icelandic studbook, WorldFengur, and the online studbook information for Standardbreds and Coldblooded trotters from the Swedish Trotting association. The datasets included 509 008 Icelandic horses (born between 1860-2020), 504 Standardbred stallions, and 125 Coldblooded trotter stallions (both born between 1990-2020). In the breeds, individuals with skewing sex ratios among their offspring (skewing horses) were identified using chi-square tests. For Icelandic horses the heritability was estimated with a parent-offspring regression, using the proportion of male offspring of fathers and sons. Families were identified using the hundred most skewing Icelandic stallions and as preparation for future research a new protocol for real-time PCR (qPCR) analysis was developed and tested, to identify sex ratios in horse semen. The proportion of males registered in the complete Icelandic horse dataset was 43.7%, whereas it was 49.0% for the last 10-year period studied. For Standardbreds and Coldblooded trotter stallions, the proportion of male offspring was 48.9% and 50.5%, respectively. Multiple skewing horses were identified in all breeds, 16.0% in Icelandic stallions, 5.8% in Icelandic mares, 11.5% in Standardbred stallions, and 6.4% in Coldblooded trotter stallions. In Icelandic horses and Standardbreds, female skewing was more common than male skewing. The heritability of the proportion of male offspring in Icelandic horses was estimated at 0.29 (SE: 0.08), but the sensitivity of this estimate to the number of offspring of father and sons required was high. Five female-skewing families were identified containing between 31 and 513 significantly skewing horses. Lastly, qPCR experimentation was performed using four primers. The Y-primer did not result in an amplification product, however the X and two autosomal controls were successful and usable on purified (without somatic cells) and raw horse semen. The difference between sex ratio and proportion of female-skewing horses in Icelandic horses and Standardbreds might be explained by incomplete recordings especially in the past, and the existence of meat horses in the Icelandic horse pedigree. These meat horses cause incomplete registration of offspring, most often males. However, the extent of this bias to the analyses is still unknown. In further analyses, meat horses need to be excluded and some more data editing performed. The heritability estimates may be improved by using a better fitting model. In future, it would be interesting to investigate sex ratios in horse semen, to compare female-skewing and male-skewing stallions to a control, to investigate where in development skewing of sex ratios start. Lastly, whole genome sequencing of skewing individuals can give us more information on the genetic cause, with the aim to identify genetic elements causing distorted sex ratios.
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