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Neoteny amphibian
Science (Sc380)
Tribhuvan Vishwavidalaya
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Neoteny in Amphibians
Two terms, paedogenesis and neoteny (coined by Kollwann) are often used as synonyms which is not correct. In fact they stand for two rather basically different evolutionary developmental processes. Neoteny refers to the retention of a larval or embryonic trait in the adult body. Familiar examples are retention of embryonic cartilaginous skeleton in adult in Chondrichthyes; and the larval gills in some adult salamanders. Paedogenesis or paedomorphosis refers to development of gonads and/or production of young ones by an otherwise immature, larval or preadult animal. The examples are scattered in several groups or animals (e. gall fly, liver fluke, salamanders). Thus, whereas neoteny emphasizes the retention of embryonic or larval features in the adult body and paedogenesis stresses precocious development of gonads in larval body. Amphibian examples. Some aquatic larval urodeles delay or fail to metamorphose, yet become sexually mature, mate and produce fertile eggs. Do they represent neoteny or paedogenesis? The distinction between these two processes becomes blurred or overlapped in these amphibians. These animals may either be looked upon as adults, which retain certain larval characteristics not having metamorphosed. This is neoteny. Or, they may be considered larvae in which reproductive organs develop precociously. This is paedogenesis.
Axolotl. Classical and most informative examples of neoteny and paedogenesis among Amphibia (vertebrates) are furnished by Ambystoma (or Amblystoma). A. maxieanum lives in Lake Xochimilco in the highlands of Mexico and the closely related. A. tigrinum (tiger salamander) in high altitudes of Colorado (North America). Ordinarily they go through typical gilled aquatic larval stages, then metamorphosis, to transform into adult air-breathing land forms. However, under certain circumstances, the larvae do not metamorphose, retain their gills and aquatic habitat but become sexually mature. This sexually mature but morphologically immature, larva Stage with external gills is called an axoiotl. The name was given by natives of Mexico who captured them for food. In Aztec axoiotl means "servant of water". At one time axolotls were considered a separate genus, Siredon, S. axoiotl, S. mexicanum, S. pisciformis etc. (Wagler, 1830). However, in 1865, some axolotls brought to'' Paris in France discarded their gills and fins and underwent metamorphosis to become adult terrestrial salamanders. Axolotls have been important laboratory experimental animals for a century, for most of them now used worldwide have descended directly from those sent to Paris in 1864 and 1868 Axolotl (Ambystoma mexicanum)
exclusively responsible for arrest of metamorphosis. So the existence of other factors, internal and physiological becomes apparent Intrinsic factors Many experimental evidences have been advanced by different investigators. Gressner (1928) was of the opinion that insulin hormone inhibits metamorphosis. But recent researches incline to reveal that' metamorphosis is primarily influenced by (i) varying threshold levels of thyroxin and its analogues and (ii) by the degree of responsiveness of the larval tissues to the hormones. During early premetamorphic stage in amphibian development, the level of thyroxin is kept very low in the body by genetic mechanism (Etkin, 1968). Etkin and his coworkers have also established the role of prolactin on metamorphosis. They have shown that the level of prolactin which acts as an inhibitor in the overall control of metamorphosis remains high at this time. In the light of modern genetics it may be suggested that the structural genes guiding the synthesis of thyroxin are "switched off* by some operator genes, whereas, the genes guiding the formation of prolactin are "switched on". In such condition, hypothalamus becomes sensitive to the available level of thyroid hormone in the blood stream. The neurosecretory apparatus of the hypothalamus produces a substance called thyrotropin releasing factor (TRF). TRF stimulates the anterior lobe of pituitary to produce thyroid stimulating hormone (TSH), which in turn enhances the rate of thyroid secretion. As the level of TSH rises during protnetamorphosis, the level of prolactin suddenly falls, so the metamorphosis starts. Poor secretion of thyroid glands and the irresponsiveness of the larval tissues to the hormone are responsible for ncotcny. In amphibian development the tadpole larva undergoes progressive metamorphosis and transforms into an adult. This is a normal occurrence
in amphibians. But deviation from the normal pathway of development is found in the life cycle of many urodeles. Such deviated pathways of development in axolotls due to extrinsic as well as intrinsic environmental factors may be regarded as "canalisation", i., buffering of development against environmental change. Neoteny is looked upon as a consequence of adaptations to neighbouring environments where retention of larval gills and other larval features may be advantageous. G. Noble (1954) regarded that the retention of larval features during sexual maturity has nothing to do in the phytogeny of the amphibians. This is quite evident from the heterogenous characters of the I'erennibranchiata where all the neotenous species are included. Neoteny as such may have some importance in the individual groups. Types of neoteny. Neoteny is- partial when metamorphosis is delayed due to temporary ecological or physiological changes in environment. It is shown by tadpoles and larvae tiding over winter. Intermediate neoteny is shown by axolotls which also reproduce sexually but undergo metamorphosis in suitable conditions. Under experimental conditions in laboratories, it is possible to produce either axolotls or transformed individuals. The extreme or total neoteny is shown by several perennibranchiate salamanders such as Necturus, Siren and Proteus. They remain larval throughout. Even treatment with thyroxine fails to induce metamorphosis; the tissue response is absent. Significance of neoteny. Weismann (1875) thought neoteny to be a case of retarded evolution or atavism, that is, reversion to ancestral condition. However, this is now regarded to be of secondary specialization, a physiological adaptation of advantage. This is also proved by the great heterogeneity of all neotenous perennibranchiate forms.
Neoteny amphibian
Course: Science (Sc380)
University: Tribhuvan Vishwavidalaya
