Why is asexual reproduction rare in chordates

This is a tricky question, since the origin and maintenance of sexual reproduction is itself complex (see Holiday 2006. Bioessays 28:1123; Zimmer 2009. Science 324:1254.)

What is clear (apparently) is that while only about 0.1% of animal species are known to be asexual, more than 1% of angiosperms are (Vrijenhoek, R.C. 1998. Animal Clones and Diversity: Are natural clones generalists or specialists? Bioscience 48:617.)

At the same time, it is also clear that asexual reproduction, known as parthenogenesis (from the Greek meaning virgin birth) has evolved a number of times, independently within sexually reproducing chordate species.

In fish and frogs, there are two major versions of asexual reproduction.

Surprisingly enough, like sexual reproduction both are dependent upon a male

In gynogenesis a male sperm activates a diploid egg, but its genome (DNA) is discarded.

In hybridogenesis the sperm also activates the egg, but its DNA is not always lost, it can be incorporated into the genome.

An asexual Poelcilia formosa female (upper) "stealing" sperm from a courting (and sexually reproducing) P. mexicana male (lower). Photograph by Manfred Schartl - adapted from Rogers & Vamosi. 2010.

Sometimes, gynogenesis is "leaky"; some male DNA is retained, so even asexual species may not be completely isolated, genomically, from their neighbors (see Stock et al, 2010. Mol. Eco. 19:5204 and references therein.)

According to Stock et al, all asexual vertebrates arose by hybridization between organisms of two different (but related) species.

Why are asexual animals more difficult to evolve than plant asexuals?

The answer to this question is not completely clear, but it may have something to due with the constraints associated with the developmental program in animals compared to plants.

Remember that it is often possible to generate a new plant from a leaf or root cutting.

While there is regeneration in a number of animals, it is much more limited.

While a newt may regenerate a severed limb, the limb will not regrow a newt.