There is a fascinating new paper dealing with the origin and evolution of sperm production in animals.
Chirag Shah, M. J. W. VanGompel et al. Widespread Presence of Human BOULE Homologs among Animals and Conservation of Their Ancient Reproductive Function. PLoS Genetics 2010. Open access.
While sexual reproduction is widespread among animals, it remains enigmatic to what extent sexual reproduction is conserved and when sex-specific gametogenesis (spermatogenesis and oogenesis) originated in animals. Here we demonstrate the presence of the reproductive-specific protein Boule throughout bilaterally-symmetric animals (Bilateria) and the conservation of its male reproductive function in mice. Examination of Boule evolution in insect and mammalian lineages, representing the Protostome and Deuterostome clades of bilateral animals, failed to detect any evidence for accelerated evolution. Instead, purifying selection is the major force behind Boule evolution. Further investigation of Boule homologs among Deuterostome species revealed reproduction-specific expression, with a strong prevalence of testis-biased expression. We further determined the function of a deuterostomian Boule homolog by inactivating Boule in mice (a representative mammal, a class of Deuterostomes). Like its counterpart in Drosophila (a representative of the opposing Protostome clade), mouse Boule is also required only for male reproduction. Loss of mouse Boule prevents sperm production, resulting in a global arrest of spermatogenesis in remarkable similarity to that of Drosophila boule mutants. Our findings are consistent with a common origin for male gametogenesis among metazoans and reveal the high conservation of a reproduction-specific protein among bilaterian animals.
The authors argue that no positive selection is apparent in the Boule gene but only purifying selection.
... the low Ka/Ks [non-synonymous/synonymous mutation] ratio suggests that purifying selection was responsible for the strong functional constraint on the entire protein, making Boule an exception to the rapid evolution commonly seen in reproductive genes.
In other words: it is a key component of the reproductive system in males across animal species that cannot be easily altered without causing the collapse of the system.
They also tested for gene expression of the Boule gene in male and female reproductive systems across the Bilaterian taxon (animals with front and back sides), finding that this gene is only manifest in the male reproductive system, unlike DAZL, which manifests in both genders among the species it does exist (most vertebrates).
Another finding is that animals (mice in the experiment) with mutant Boule gene are perfectly normal except that they are infertile. They even have a normal mating behavior. The only apparent difference is in the testes, which are somewhat smaller. Similar effects were found in fruit flies.
However, inside the testes, the effect of the mutant Boule gene is dramatic, totally impeding spermatogenesis.