Evolution and phylogeny of gonad morphology in bony fishes

Gonad morphology at the gross anatomical or histological levelshas long been studied by fisheries biologists to identify annualreproductive cycles and length of breeding season, among othergoals. Comparative surveys across vertebrate taxa have not beendetailed enough, however, to describe fully the differencesand 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 agerminal epithelium composed of somatic and germ cells in bothmales and females. In females, the germinal epithelium linesthe ovarian lamellae. In males, arrangement of the germinalepithelium into compartments varies among osteichthyans: basaltaxa have an anastomosing tubular testis, whereas derived taxahave a lobular testis. The lobular testis is proposed as a synapomorphyof the Neoteleostei. The annual reproductive cycle is hypothesizedto be the source of morphological variation among testis types.Elongation of germinal compartments during early maturationmay result in a transition from anastomosing tubular to lobulartestes. In all male atherinomorphs surveyed, spermatogonia arerestricted to the distal termini of lobules rather than beingdistributed along the lobule; there is an epithelioid arrangementof Sertoli and germ cells rather than a germinal epithelium.Arrest of the maturation-regression phases is hypothesized tolead to formation of the atherinomorph testis. Atherinomorphsalso have a distinctive egg with fluid, rather than granular,yolk. Variation among germinal epithelia is interpreted in adeveloping phylogenetic framework to understand evolution ofgonad morphology and to propose gonad characters for phylogeneticanalyses.