Synaptonemal complex analysis of an autosomal trisomy in the horse.

The research paper explored the occurrence and effects of trisomy, with a particular focus on chromosome 28, in male horses. The study observed the implications on formation and degeneration of spermatocytes and spermatids, leading to infertility in the studied horse.

Synaptonemal Complex and its Analysis

• The study started by analysing the synaptonemal complex using electron microscopy in a male horse that exhibited trisomy 28, a condition whereby the horse has an extra copy of chromosome 28.
• The synaptonemal complex is a protein structure that develops between two homologous chromosomes during the first stage of meiosis. This makes it critical in sexual reproduction and thus exploring it forms the basis of understanding the genetic implications of trisomy in horses.

Trivalent and Bivalent Chromosomes

• The research observed either a trivalent chromosome, implying three chromosomes grouped together, or a bivalent-and-univalent combination in the primary spermatocytes of the horse.
• It was seen that two of the three chromosomes within the trivalent were almost completely paired with each other, but only partially with the third chromosome.

Chromosomal Pairings

• The study found that about half of the analysed spermatocytes exhibited heterologous pairings or associations between the free axis of the trivalent and the sex bivalent chromosomes. Heterologous pairing refers to the linkage between different sorts of chromosomes.
• These pairings were largely preserved even into the stages of diakinesis-metaphase I, demonstrating a long-lasting chromosomal interaction influenced by the trisomy.

Horse’s Fertility

• Additionally, the research identified proximal asynapsis, an incomplete or partial pairing of chromosomes, in one of the autosomal bivalents of most spermatocytes. These were also often paired with the trivalent or the sex bivalent.
• Such chromosomal interaction leads to formation of abnormal spermatocytes, which in turn results in production of defective sperms and eventually, infertility. This was evident in the studied horse that demonstrated azoospermy, a condition characterized by the absence of motile sperm in semen.
• The researchers attributed the horse’s azoospermy, at least partly, to the cellular degeneration observed at the spermatocyte and spermatid stages, caused by the trisomy.

The study thus offers insights into the genetic complications of trisomy in horses, with potential implications for broader mammalian trisomy research.