Gamete Biology

Transcriptome Analysis of Bull Semen with Extreme Nonreturn Rate: Use of Suppression-Subtractive Hybridization to Identify Functional Markers for Fertility

Abstract
Spermatozoa are terminally differentiated cells produced during the complex process of spermatogenesis. Although the role of their residual RNA content is still being debated, this transcriptome may represent a fingerprint of spermatogenesis quality. In this study, we undertook differential transcript profiling of spermatozoa from fertile bulls with extreme non-return rates: a low-fertile group and a high-fertile group. Using the suppression subtractive hybridization technique in combination with macroarray analysis, we also identified novel genes. Both extreme NRR index groups retain redundant identity, such as ribosomal and mitochondrial sequences, at a statistically significant level. An elevated number of 12S, 18S, and Large Chain R rRNA gene copies were found in low-fertile bulls and validated in spermatozoa by quantitative RT-PCR for a small cohort of bulls with known fertility index. Whereas the high-NRR library exhibits a large proportion (29%) of transcripts associated with known functions (e.g. metabolism, signal transduction, translation, glycosylation and protein degradation), only 10% of the low-NRR sequences do. This difference is also conveyed by two other categories, 17% Bovine Genome and 48% Unknown in the high-NRR library, whereas the percentages are 3% and 80%, respectively, in the low NRR library. Some of the Unknown transcripts are similar to expressed sequence tags detected in the male reproductive organ of certain plants and retain homology to a putative human protein. Whereas the individual transcriptome profiles may be useful in fertility assessment, these findings also suggest cross-species conservation and could contribute to a better understanding of spermatogenesis and provide new insights into idiopathic infertility.

Key words: Gamete Biology • Sperm • Spermatogenesis