Histological analysis of spermatogenesis within non-mammalian vertebrates.

Data on spermatogenesis in vertebrates is vast and within mammals information on the morphology, ontogeny, molecular biology, and physiological mechanisms of sperm development is accelerating at a continuous pace. There is no better example of the data being generated on spermatogenesis than this fine journal, which is dedicated just to that process. In contrast, non-mammalian vertebrates historically have not received the intense attention that their synapsid kin have. This lag in data generation is unfortunate, especially since these basal taxa are the most speciose and have the greatest diversity of reproductive strategies within both sexes within the Vertebrata. For example, squamate reptiles currently number over 9,000 different species and within lizards and snakes only a handful of studies have been performed on the fine morphology of germ cells as they pass through the phase of spermatogenesis. Because of this lack of data, our understanding of the mechanisms of endocrine control, seasonality, germ cell output, evolutionary implications, and reproductive strategy influences on spermatogenesis are not well understood. Key features such as what controls refractory period, recrudescence, or continuous versus seasonal spermatogenesis is unknown. Basic data on the morphology of germ cell development and the ultrastructural features of spermatogonia, spermatocytes, and spermatids would allow one to make better inferences on the control and mechanisms involved in the physiological timing of spermatogenesis in these fantastic vertebrates. Also, many of the basal taxa of vertebrates spend a large percentage of their live history in aquatic environments. Many of these species also have wide ranges and are quite abundant, which lends them to being excellent models for toxicology studies involving pesticides and other aquatic toxins. Another major area of interest within the last decade is the use of morphological data involving sperm development in nontraditional phylogenetic analysis. Spermatozoal and spermiogenic ultrastructural data seem to provide large enough data sets to provide some hypotheses on the phylogenetic relationships between vertebrate taxa (see for example, Jamieson). Thus, we dedicate this special issue to the morphology and ultrastructural features of spermatogenesis in several non-mammalian species of vertebrates. Our hope as authors is to stimulate the interest of reproductive morphologist in the process of spermatogenesis in basal vertebrate taxa. Many of the authors in this issue have dedicated most of their professional careers carving out our basic understanding of germ cell development in fishes, amphibians, reptiles, and birds. The authors and the Special Issue Editor would like to personally thank C. Yan Cheng, who allowed us to put this special issue together. We hope that the information provided here stimulates research interest in spermatogenesis in all the major vertebrate lineages and provides background data that provide models for the study of spermatogenesis in non-mammalian vertebrates.