In this article we will discuss about the meaning and types of pademorphosis.
Meaning of Paedomorphosis:
The phenomenon of attainment of sexual maturity leading to reproduction in an arrested larval stage (pre-adult stage) is called Paedogenesis (Gk. pais; child; morphe form). The term first used by Von Baer (1866). Neoteny (Gk. neos, young, teinein, to stretch), a term first introduced by Kollman in 1882 and a phenomenon in which the larvae become sexually mature while still retaining gills and some other larval features.
The paedomorphosis (Gk. pais, child; morphe, form) is the retention of ancestral juvenile characters in the late developmental stages of descendants. The affected individuals are called paedomorphic. But Kardong, 2002 refers to Paedomorphosis as an individual which is larval in anatomy but it becomes sexually mature and neoteny is a special case of paedomorphosis where sexual maturity occurs but somatic development slows, allowing juvenile features to persist.
Garstang proposes the term paedomorphosis in 1928. In older literature the alternative name for paedomorphosis is neoteny.
H. Schindewolf (1936) used the term proterogenesis which is practically synonymous with neoteny. Progenesis (Gk. pro, before, genesis, be born) refers to the precocious sexual maturation of an individual that is still in a morphologically juvenile stage, leading to paedomorphosis.
De Beer (1951) favours to use the broader term Heterochrony which is any evolutionary changes in the relative rates of development of characters during ontogeny. McFarland et al., (1985) use the term paedogenesis, neoteny and pro-genesis for the attainment of sexual maturity in an arrested larval animal.
Occurrence:
The paedomorphic animals occur in the families of Proteidae and Cryptobranchidae and some species of Ambystoma.
Types of Paedomorphosis:
There are two types of paedomorphosis found in urodeles.
i. Obligatory Paedomorphosis:
Some species of urodeles or larval stages always remain mature permanent larval stages. They do not metamorphose at any time. The developing tissues fail to respond to the thyroid hormone (T4), when in other forms the tissues respond to the thyroid hormone and metamorphose into adult. In obligatory paedomorphosis, the genes for transformation during metamorphosis have become suppressed.
Examples:
Some populations of Cryptobranchus alleganiensis, after complete metamorphosis, retain a single gill-slit that reflects the ancestral larval feature. So it may be considered a paedomorphic species. Necturus maculosus live permanently in ponds and lakes of U.S.A.
They have external gills, mid-dorsal caudal fin and the lung is highly reduced. Proteus, a blind urodele, lives in European caves with external gills and no pigments in the body. Siren of U.S.A. shows almost larval features with external gills and caudal mid-fin.
Amphiuma of United States have no external gills but larval gill-slits. They have vestigial limbs but without eyelids. They are all obligatory paedomorphic individuals. They do not metamorphose and cannot be induced by T4 and are genetically fixed. The Mexican axolotl of Ambystoma mexicanum which lives in Lakes Xochimilco and Chalco of Mexico, representing a typical example of paedomorphosis.
The ecological conditions of the montane lakes help the axolotl population to become sexually mature in the larval stages and lead a permanent larval stage and the genes for transformation have not entirely disappeared, probably have become suppressed.
ii. Facultative Paedomorphosis:
The axolotl larva of some urodeles, e.g., some species of Ambystoma of north western part of U.S.A. and Triturus exhibit paedomorphosis. The larvae become sexually mature and breed but they can metamorphose and change into adults when the available conditions are changed. They are not the permanent larvae like Necturus or Proteus.
In this case the hormone of thyroid gland, thyroxine (T4) induces the metamorphosis. When they remain in aquatic condition they possess external gills, gill-slits, median dorsal fin and are able to reproduce but when water is dried up or changes the environmental condition, they metamorphose into adult individuals losing its larval features. Thus this type of paedomorphosis is called facultative paedomorphosis.
The paedomorphic forms are found mainly among urodeles. A list of paedomorphic urodeles is given here. Paedomorphosis is seen in the families like Cryptobranchidae paedomorphic aquatic, semi-larval forms; 2 genera, 3 species; Cryptobranchus alleghaniensis, Megalobatrachus (= Andrias) Dravidians, M. japonicus); Proteidae (paedomorphic aquatic salamanders; 3 pairs of external fringed gills and 2 pairs of gill openings; 2 genera, 6 species);
Ambystomatidae (many paedomorphic forms; Ambystoma talpoideum of eastern North America and several species of Western United States); Sirenidae (aquatic salamanders with external gills; 2 genera, 3 species; e.g., Pseudobranchus and Siren); Salamandridae (e.g., Notophthalmus perstriatus, N. viridescens, Triturus alpestris, T. cristatus); Plethodontidae (e.g., Eurycea multiplicata, E. latitans; Typhlomolge rathbuni, T. robusta).
The paedomorphic forms can be recognised by the presence of larval characteristic features, such as the absence of eyelids and retention of external gills (in some cases) and functional lateral line system. The larval tooth and bone patterns also help to recognise the paedomorphic forms. Significance of paedomorphosis
As a paedomorph, axoltl offers excellent opportunities for understanding the role of larval characteristics in controlling or limiting the evolution of adult features.
Garstang (1928) proposed that vertebrates originated from tunicate-like larvae that failed to metamorphose but developed reproductive organs, and he termed this evolutionary process as paedomorphosis. Garstang’s term may be applied to include phylogenetic progress by neoteny and deviation.