Thursday, October 21, 2010

The Yolk on Egg Size

The article “Early development of Ensatina eschscholtzii: an amphibian with a large, yolky egg”, posted in EvoDevo Journal by Collazo and Keller, looks at early development in Ensatina eschscholtzii. This species is also commonly known as Ensatina, and is a plethodontidae (lungless) salamander that can be found in chaparrals, coniferous forests, and oak woodlands, throughout North and South America (there are seven total subspecies of Ensatina). Among amphibians there is some variability in the early processes of development that have once been thought to be derived during evolution. It is now believed that increased egg size could play a factor in affecting the early developmental functions such as gastrulation and cleavage patterns. Salamanders in the family Plethodontidae have very large yolky eggs that vary in size, making them a good candidate for observation regarding this theory.
As seen in hagfishes, elasmobranches, teleosts, coelacanths and amniotes, once an egg reaches a certain size it shifts from holoblastic to meroblastic cleavage. This shift in cleavage varies in different animal taxa. If different groups of amphibians were to be studied it might be discovered that these taxa also respond similarly to increases in egg size. In this study Egg size and embryonic developmental rate as well as cleavage pattern, which appeared to be holoblastic, were all noted to be different than that of other amphibians. Blastulation, gastrulation and formation of the embryonic disc were also observed and noted to be unlike the corresponding characteristics in other amphibians.

This paper thus concludes that large egg size and a large amount of yolk explain all of the developmental differences observed in Ensatina eschscholtzii.

EvoDevo Journal


  1. So, the difference in early embryonic stage in this salamander is due to the size of the yolk? What was it about the large egg size or large amount of yolk that caused the differences?

  2. The authors seem to argue that what we know about early amphibian development is based on a few well known species, such as the African clawed frog, Xenopus. They hypothesized that when eggs become very large, which they often are in plethodontids, that there are some specific changes in cleavage pattern and early embryogenesis. I think this is due to the constraint of growing the embryo on a very large yolk.

    One take home message seems to be that we often ignore the diversity of biological mechanisms when we focus too hard on a small number of model species.

    It is also interesting to note that the large yolk size in plethodontid salamanders may correlate with the evolution of direct development in this group.