Photogallery: tiny chameleons
A study led by the Higher Council for Scientific Research (CSIC) has analyzed theevolution of reproductive forms of amphibiansand the passage of theegg laying from the aquatic to the terrestrial environment. The work, published in the magazineEvolution, reveals that the conquest of the terrestrial environment implied an increase in the size of the eggs, as well as a reduction in the size of the adults and the number of eggs per laying.
As explained by CSIC researcher Iván Gómez-Mestre, from the Doñana Biological Station, amphibians present the greatest variety of forms of reproduction of all terrestrial vertebrates. “In this work, we show that ancestral amphibians already reproduced by aquatic eggs that gave rise to tadpoles, as more than half of frog species still do today. This is a system that has been maintained for longer. 200 million years old, “he points out.
The other half, the one that has not maintained the aquatic environment for egg laying, underwent multiple independent evolutions, and some species have reached direct development, in which the tadpole phase disappears. “Studying these changes in the forms of reproduction is important, among other things, because the rest of the terrestrial vertebrates (reptiles, birds and mammals) evolved from some group of amphibians with direct development”, adds Gómez-Mestre.
According to this study, direct development, in which a frog or toad is born directly from the terrestrial egg, has evolved from the aquatic condition to the terrestrial condition both through intermediate stages and in a single jump. In rapid transitions, a development with a free-living larval stage would have occurred to an integral development, in which the young are formed within the egg and are completely dependent on the initial maternal supply.
Larger eggs, smaller adults
The team responsible for this work has shown that the passage to land was associated with an increase in the size of the egg , possibly to reduce the risk of desiccation. In addition, there is a balance in reproductive investment, so that if the size of the eggs increases, each female lays fewer of them.
This work shows that the transition to terrestrial reproduction also led to a decrease in adult size . The fact that smaller females laid larger eggs also forced a reduction in laying and favored the emergence of parental care. However, as they clarify in the article, the different modes of reproduction in amphibians are still largely dependent on water.
