Evolution and phylogeny of gonad morphology in bony fishes

Gonad morphology at the gross anatomical or histological levels has long been studied by fisheries biologists to identify annual reproductive cycles and length of breeding season, among other goals. Comparative surveys across vertebrate taxa have not been detailed enough, however, to describe fully the differences and similarities among gonads of bony fishes and other vertebrates, and to use gonad morphology in phylogenetic systematic analyses. An emerging constant among vertebrates is the presence of a germinal epithelium composed of somatic and germ cells in both males and females. In females, the germinal epithelium lines the ovarian lamellae. In males, arrangement of the germinal epithelium into compartments varies among osteichthyans: basal taxa have an anastomosing tubular testis, whereas derived taxa have a lobular testis. The lobular testis is proposed as a synapomorphy of the Neoteleostei. The annual reproductive cycle is hypothesized to be the source of morphological variation among testis types. Elongation of germinal compartments during early maturation may result in a transition from anastomosing tubular to lobular testes. In all male atherinomorphs surveyed, spermatogonia are restricted to the distal termini of lobules rather than being distributed along the lobule; there is an epithelioid arrangement of Sertoli and germ cells rather than a germinal epithelium. Arrest of the maturation-regression phases is hypothesized to lead to formation of the atherinomorph testis. Atherinomorphs also have a distinctive egg with fluid, rather than granular, yolk. Variation among germinal epithelia is interpreted in a developing phylogenetic framework to understand evolution of gonad morphology and to propose gonad characters for phylogenetic analyses.