Seasonal functional relevance of sperm characteristics in equine spermatozoa

Abstract

A group of stallions with different reproductive indexes were used to study seasonal variations in sperm quality (Equus caballus). Semen samples were collected from late September to July and analyzed according to four seasonal periods: late September–December, January–March, late March–May, and June–July. Parameters monitored included sperm concentration, sperm motility, sperm morphology, sperm viability, acrosomal status, plasma membrane stability, and sperm mitochondrial membrane potential. Overall, seminal parameters monitored are affected mostly by time period, followed by animal and lastly by fertility, stressing the importance of individual variations in out-bred animal models. The analysis of multiple ejaculates from the same animals showed clear seasonal-based differences (P < 0.05) with poor semen quality in winter and a noticeable improvement in sperm quality with increasing photoperiod. Better semen quality was observed between late March and May. Interactions between month period, animal, and fertility were evident (P < 0.05) for sperm concentration, head and tail sperm anomalies, and acrosomal integrity. Thus, it may be advisable to adjust the use of stallion semen according to seasonal variations.

Introduction

Similarly to other mammals, equines restrict their reproductive efforts to the time of the year when conditions are most favorable for the successful weaning of offspring [1]. The seasonal change in day length is the main, though not the only, cue responsible for the circannual rhythm of reproductive activity [2]. On the whole, the fertile period takes place from April to September in the Northern Hemisphere [3]. However, in the majority of equine breeding associations, the breeding season is officially established from February to June, forcing breeders to breed mares as early as possible in the year. To what extent the quality of semen collected early in the breeding season can affect reproductive indexes is not known.

In the horse, male gametogenesis, reproductive tract function, and resulting sperm functionality are likely to be optimal during the breeding season. Throughout the winter there seems to be a decrease in sperm production [4], and seasonal fluctuations have also been observed for sperm viability and motility [5], [6]. According to Guillaume [7], fertility in stallions may be maintained at a reduced level outside the breeding season. Moreover, seasonal breeders from latitudes between 30°N and 40°N do not exhibit the same dramatic seasonal changes in reproductive activity as animals from higher latitudes [8]. Nevertheless, no data exist on seasonal variation in semen quality in horses from those latitudes.

The in vivo fertilizing ability of a given sperm sample is dependent on the interplay of a variety of attributes representing different features required for fertility. Sperm motility, in particular, is considered to be a fundamental laboratory test for assessing an ejaculate [9]. Further aspects of sperm functionality that can be probed include sperm nuclear DNA integrity, acrosomal contents, plasma membrane stability related to capacitation, and mitochondrial activity in the sperm midpiece, among others (for a review, see [10]).

In this work, we have attempted to characterize seasonal variations in equine semen quality, namely monitoring sperm concentration, motility, morphology and viability, as well as acrosomal status, plasma membrane stability, and mitochondrial membrane potential. Furthermore, and unlike the majority of previous studies, we have attempted to average out what sperm characteristics are functionally important by obtaining a series of samples from the same males during different seasonal periods.