Sexual Maturity in the Larval Body of Salamanders !

Salamander larvae acquire sexual maturity before their metamorphosis into adults. So, they become sexually adult before their complete physical adultness !
Two parallel concepts are persisting through the Paedogenesis-Neoteny spectrum. One concept is related to the attainment of sexual maturity in larval stage before the completion of metamorphosis and the second concept is related to the delaying of metamorphosis by retaining the larval external morphology. New terms like Progenesis, Partheno-paedogenesis have been introduced by different scientist in this context.

DIFFERENCE PAEDOGENESIS AND NEOTENY

The term Paedogenesis is applied only when larvae are sexually matured as a result of accelerated reproductive development. But, the term Neoteny is applied when the sexual development is not precocious; rather the somatic development is delayed as a result of which reproductive development is apparently appeared faster. In 1977, Gould suggested that Paedogenesis is not a common cause of larval reproduction in Salamanders rather the actual cause of their larval adultness is Neoteny. All members of four families of Salamanders are neotenic.
Therefore in Paedogenesis animals are developing their sexual features quicker than somatic development. But in Neoteny, animals are developing their sexual features normally, but the somatic development is delayed by some reasons of endocrinal problems. Neoteny is mainly found in the members of Order Urodela of class Amphibia.

PATTERNS OF NEOTENY

Neoteny may be partial or complete. In case of partial Neoteny metamorphosis retardates beyond the normal growth period. This type of Neoteny is found in the tadpole larva of Rana esculenta, Alytes obstericus etc. This is caused due to the formation of Thyroid glands. In case of complete Neoteny animals retain the gills but become sexually developed as found in Urodeles. This type of Neoteny is found in the tadpole larva of Ambystoma mexicanum, Triton vulgaris etc.

SALIENT FEATURES OF THE AXOLOTL LARVA

Axolotl larva is found in the genus Ambystoma.The general anatomical peculiarities of the Axolotl larva is similar to that of its adult, however , a number of features serve to distinguish the larvae from adults.

1. Elongated body with a laterally compressed caudal fin.
2. Three pairs of external gills on the posterolateral sides of the head.
3. Distinctive larval dentition and presence of vomerine teeth.
4. Well developed eyes.
5. Presence of scleral cartilage that disappears during metamorphosis.
6. Four pairs of gill openings.
7. Poorly developed limbs.

REASONS FOR NEOTENY

a) Temperature plays an important role for occurring Neoteny. Some larvae were exposed to temperature below than 5°C could not form their complete metamorphosis. On the other hand, some larvae were exposed to higher temperature also could not form complete metamorphosis.

b) The fault in proper functioning of Hypothalamic-Pituitary-Thyroid axis is the main cause of Neoteny. If a deficiency occurs in the secretion of T.S.H. from the adenohypophysis, with the subsequent insufficient secretion of the main Thyroid hormones then Neoteny would be happened.

c) The range of body size and duration of the larval stage are determined by a minimum body size that should be attained and a maximum body size that cannot be exceeded during metamorphosis. Between those two size thresholds, the endocrinological initiation of metamorphosis is expected to happen.

d) The deficient activity of Thyroid glands and reduced sensitivity of the larval tissue to the Thyroid hormones may lead to the happening of Neoteny.

e) It was observed that the mRNA for Thyroid hormone receptor is absent in Necturus (Mud puppy) and thus cannot be induced by tri-iodo thyronin (T₃).

f) In Salamander heterochrony is caused by gene mutation in the induction competence systems. Other heterochronic phenotypes are caused by the heterochronic expressions of certain genes. Research works are going on.

FURTHER READINGS MAY BE DONE FROM :

1. Gould, 1977.

2. Dwellman and Trueb, 1986.

3. Gilbert, 1997.

4. Wilber and Collins, 1973.