Developmental endocrinology of the dipnoan, Neoceratodus forsteri

American Zoologist, Dec 1997 by Joss, Jean M P, Rajasekar, P Sylvia, Raj-Prasad, R Ashni, Ruitenberg, Kirsty

Developmental Endocrinology of the Dipnoan, Neoceratodus forsteril

JEAN M. P JOSS,2 P. SYLVIA RAJASEKAR, R. ASHNI RAJ-PRASAD, AND KIRSTY RUTENBERG

School of Biological Sciences, Macquarie University, Sydney, NSW 2109, Australia

SYNoPSIS. The development of the pineal, pituitary and thyroid glands of the extant lungfish, Neoceratodus forsteri, are being studied both morphologically and functionally. This paper presents data from hatching to 40-52 weeks for a standardised series of lungfish, bred at Macquarie University. At hatching, the pineal comprises a single organ attached to the roof of the diencephalon, with well-developed photoreceptor, supporting and ganglion cells. The photoreceptors gradually degenerate, giving way to secretory cells which contain electron dense granules. These latter are immunoreactive to melatonin antibodies and digestable with protease. The pituitary at hatching comprises a hollow ball of cells lying beneath the infundibular region of the hypothalamus. Ultrastructurally, four cell types can be distinguished by cytoplasmic granule size after the first four weeks of development posthatching. By 20 weeks, a further three cell types are recognisable. Immunogold labelling has identified corticotropes and melanotropes at four weeks and, at 20 weeks, prolactin cells, thyrotropes and somatotropes also can be identified. The thyroid is only just apparent at hatching, containing 23 follicles. The numbers of follicles increases gradually, and variably between animals, with age. Iodine uptake in methimazole-treated animals did not exceed that of controls at any of the three stages tested, indicating a lack of feedback control between thyroid hormones and pituitary thyrotropes at, or before, 40 weeks of age. Thyroid hormone receptors in the liver at 40 weeks are predominantly immunoreactive to human TRa antibodies. These findings taken together suggest that, up to 40 weeks post hatching, lungfish development is equivalent to amphibian premetamorphic development. This would be consistent with lungfish neoteny, but cannot be taken as evidence for neoteny until confirmed at later stages of development.

INTRODUCTION

Recent molecular phylogenetic studies (Meyer, 1995; Zardoya and Meyer, 1996) are providing strong evidence that lungfish are the closest living ancestor to the land vertebrates, the coelacanth being the only other living contender. Neoceratodus forsteri is the sole representative of the family most closely resembling the post-Devonian lungfish. As such, study of its development may hold the key to appreciating the origin of such novel tetrapod features as limbs and parathyroid gland. As a starting point in my study I have chosen to explore the possibility that the seemingly "primitive" features of recent lungfish may be a result of neoteny and be better explained as "larval" features, e.g., presence of unrestricted notochord and absence of marginal jaw bones.

The developmental descriptions presented in this paper form the first part of a much larger, ongoing study which seeks to address the question: Is the apparent direct development of recent lungfish actually the equivalent of premetamorphic stages characteristic of an obligate neotene? Several authors in the past (Moy-Thompson and Miles, 1971; Gardiner, 1973; Smith, 1977; Bemis, 1984) have implied paedomorphic tendencies in dipnoans. The most rigorous consideration of paedomorphosis in the evolution of lungfish is by Bemis (1984) who compared Devonian fossil lungfish with the three genera of recent dipnoans, and argued that several differences such as those of median and tail fins, endochondral ossification and cell size, are best interpreted as paedomorphic. These are all morphological features. Our study is making use of the captive breeding lungfish at Macquarie University to study the development of endocrine glands that may show similar patterns of activity to those associated with neoteny in some urodele amphibians. There have been almost no developmental endocrine studies carried out on any lungfish. Some of the early developmental studies by Semon (1901), Greil (1906, 1913), Kerr (1910, 1919), and Siwe (1926) include descriptions of endocrine glands but there have been no recent studies, using more modern techniques to indicate something of the development of function of these glands.

By contrast, there is a wealth of knowledge about the development of endocrine glands in the amphibians, particularly in those species undergoing the most dramatic metamorphoses. Briefly, the function of the thyroid has been shown to be central to amphibian metamorphosis, with several other endocrine glands (pituitary, hypothalamus, adrenal) also being involved as they in some way affect the function of the thyroid or its hormones. The major controllers of thyroid hormone action in developing tadpoles are the levels of circulating thyroid hormones, the activity of the hypothalamicpituitary-thyroid axis and, in the tissues, the number of thyroid receptors and the activities of the SD and 5D' deiodinase thyroid hormone converting enzymes (Galton, 1988; Becker et al., 1997). Where neoteny has been studied in amphibians, it has been shown to be associated with a deficiency in one or more of these parameters (Rosenkilde and Ussing, 1996). For example, neotenic ambystomatids (axolotls) show a low level of thyroid activity for most life stages. However, early larvae do display a wave of increased circulating thyroxine associated with dramatic development of the central nervous system. This increased thyroxine level does not result in metamorphosis because the 5'-deiodinase activity in the peripheral tissues necessary for conversion of thyroxine to the more active trodothyronine is out of phase with the thyroxine surge. In other urodeles, such as Necturus, metamorphosis is prevented by few or no thyroid hormone receptors in tissues which undergo metamorphosis. Other hormones, such as prolactin from the pituitary or cortisol from the adrenal gland also may be involved in suppressing thyroid-induced metamorphosis. It is the eventual aim of this study to investigate all these endocrine aspects of development in N. forsteri in order to answer the question of neoteny in lungfish.