The evolution of reproduction and the sex chromosomes

The aim of this project is to dissect the genetic cause behind distorted sex ratios in horses. This includes to develop assays to determine if there are any chromosomal abnormalities in the sex chromosome pair among parents and offspring. Such assays can potentially be used for diagnosis of reproductive problems in horses. In another part of the project, selfish genetic elements that may cause the distorted sex ratios will be investigated.

Genomes are vulnerable to selfish genetic elements (SGEs), which enhance their own transmission relative to the rest of an individual's genome but are neutral or harmful to the individual as a whole. As a result, genetic conflict occurs between SGEs and other genetic elements in the genome. 

Selfish genetic elements that distort Mendelian segregation to favor their own transmission are common in eukaryotic genomes. Segregation distortion can reduce whole organism fitness, resulting in strong counter selection for genes that suppress distorters. Such intragenomic conflicts have the potential to drive bouts of antagonistic co-evolution, i.e. the aggregation of compensatory mutations to restore sex balance. One important consequence of recurrent sex-linked conflict is its potential to drive speciation.

Preliminary data suggests that we have identified several Standardbred stallions with a distorted sex ratio among their offspring. Some sires have significantly more males among their offspring and others have more females.

Genetics behind distorted sex ratios

In this project, the student will dissect the genetic cause behind the distorted sex ratios. The first experiment will consist of developing assays (diagnostic tools) to determine if there are any chromosomal abnormalities in the sex chromosome pair (ECAX and ECAY) among parents and offspring. The development of the assays will be performed as a collaboration between Texas A&M University and SLU.

In addition to the purpose of this research project, these assays can be used in the future for diagnosis of reproductive problems in horses due to abnormalities in the sex chromosomes. A large proportion of such sex chromosome abnormalities remain undiagnosed in horses due to the lack of an affordable diagnostic tool that allows for avoiding karyotyping tests. If we identify any chromosomal abnormalities in this research project, karyotyping will be performed at Texas A&M under the leadership of Prof Terje Raudsepp.

Selfish genetic elements

If no sex chromosomal deviations are detected, we aim to map and study selfish genetic elements that may cause the distorted sex ratios.

This will be achieved by:

  1. identifying additional horse half-sib families, in multiple horse breeds, where a distorted sex ratio can be observed over at least two generations (i.e. a stallion and its father both skew the sex ratio among their offspring in the same direction) and
  2. whole genome sequencing (long-read) of these horse families. In males, the meiotic drive system usually includes two loci - both a “driver” and a “target”. Drivers are many times linked to an insensitive allele of the target to avoid self-destruction. Therefore, meiotic drive systems tend to reside in regions of the genomes with low recombination (e.g. heterochromatin or sex chromosomes).

Since we previously performed a large-scale copy number variant (CNV) study in European horse breeds, we also aim to investigate if there are any sex-determining genes within these CNV regions.

Pinpointing selfish genetic elements (sex chromosome drivers), as well as compensatory mutations to restore sex balance, could have practical implications in horse breeding. Quite frequently, molecular sexing is performed on horse embryos. In nature, the genetics underlying a meiotic drive system can lead to lineage splitting and speciation. This is because compensatory mutations may create hybrid incompatibility that could, perhaps, establish reproductive barriers.

Supervisors: Gabriella Lindgren &  Terje Raudsepp (Texas A&M University)

If you are interested in this project, please contact:
Gabriella Lindgren
Department of Animal Breeding and Genetics, SLU

Last modified: 2021-09-09