Abstract
The relationship between sperm energetics and sperm function is poorly known, but is central to our understanding of the evolution of sperm traits. The aim of this study was to examine how sperm morphology and ATP content affect sperm swimming velocity in the zebra finch Taeniopygia guttata. We exploited the high inter-male variation in this species and created extra experimental power by increasing the number of individuals with very long or short sperm through artificial selection. We found a pronounced quadratic relationship between total sperm length and swimming velocity, with velocity increasing with length up to a point, but declining in the very longest sperm. We also found an unexpected negative association between midpiece length and ATP content: sperm with a short midpiece generally contained the highest concentration of ATP. Low intracellular ATP is therefore unlikely to explain reduced swimming velocity among the very longest sperm (which tend to have a shorter midpiece).
Highlights
Sperm morphology evolves rapidly in internally fertilizing species in response to selection inside the female reproductive tract, and this has resulted in dramatic diversification across species [1]
Our analyses revealed a significant positive association between sperm swimming velocity and both head and tail—but not midpiece length—across the entire sperm sample and in the fastest 10%
Because both head and tail length are strongly correlated with total sperm length, these relationships could be driven by an overall positive association between total sperm length and swimming velocity
Summary
Sperm morphology evolves rapidly in internally fertilizing species in response to selection inside the female reproductive tract, and this has resulted in dramatic diversification across species [1]. For example, sperm must be motile to rapidly traverse the hostile environment of the vagina to gain access to sperm storage sites [4]. Costly traits, such as high swimming velocity and longevity, are likely to be crucial for success [5]. Sperm swimming velocity appears to be strongly influenced by sperm morphology, i.e. the size and shape of sperm components: the head, midpiece and flagellum. The relationship between midpiece length and swimming velocity, is more complex and differs between species (e.g. [9,10])
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