Unisexual Clones: Lizards and Corals

The fascinating and well-conceived Report by S. V. Vollmer and S. R. Palumbi (“Hybridization and the evolution of reef coral diversity,” 14 June, p. [2023][1]) shows that interspecific hybridization in corals can produce F1 offspring that “can reproduce asexually and form long-lived, potentially immortal hybrids with unique morphologies.” A similar phenomenon occurs in terrestrial vertebrates, among reptiles, but the unisexual clones are treated as species by taxonomists. The vast majority of lizards are bisexual (dioecious, gonochoristic) species. Ova will not develop until or unless they are fertilized, and the animals have Mendelian patterns of inheritance. There are also, however, a few morphologically recognizable unisexual (all-female) species that reproduce by obligatory parthenogenetic cloning. Some of these are triploid species that do not suffer from imbalances of gene activity. Phylogenetic analyses reveal that the clones are of hybrid origins and the switch from sperm-dependent to sperm-independent reproduction occurred in one generation. When successful, the ancestral bisexual species and their clonal derivatives continue to perpetuate themselves independently as distinct entities and all constitute separate species on different evolutionary tracks, even though the clonal lineages do not survive long in geological time. Interested readers can access this literature through Reeder et al. ([1][2]). 1. [↵][3]1. T. W. Reeder, 2. C. J. Cole, 3. H. C. Dessauer , Am. Mus. Novit. 3365, 1 (2002) (available at ). [OpenUrl][4][CrossRef][5] # Response {#article-title-2} The fundamental evolutionary biology of the coral hybrids we studied in the Caribbean is very similar to the existence of unisexual and parthenogenetic animals like Cnemidophorus lizards, skinks, and live bearing fish [see Bell ([1][6]) for a comprehensive list]. In all cases, the hybrid clones produce a descendant line identical to their single ancestor, and independent hybrid lines give rise to potentially competing “experiments” in interspecies hybridization ([2][7]). Successful lines thrive, but perhaps not for long if the niche to which they were by chance adapted disappears. Perhaps the biggest difference is that the vertebrate hybrids must be able to develop parthenogenetically—they still produce eggs that must successfully complete full development ([1][6]). By contrast, colonial animals like corals can propagate without parthenogenesis through colony fragmentation and regrowth. Each new “generation” need not pass through the germ line, and for these hybrid corals, the soma reigns supreme. Furthermore, because even sexually competent corals can clone themselves in this manner, coral hybrids need not invent new developmental machinery to persist without sexual reproduction. Clearly, the evolutionary scenario in which interspecific hybrids become secondarily clonal, either parthenogenetically or vegetatively, has played out numerous times in a diversity of organisms. Because they are endowed with the genomes of their parent species fixed in a hybrid state, the future of these clones is dim and their existence transient. Thus, they are genetic lines with no past and no future beyond that afforded a temporary winner in the ecological lottery. Because these hybrid clones make a virtually instantaneous jump to open ecological niches—in effect side-stepping the usually long process of speciation—their real evolutionary potential is to provide us with a window into processes of ecological diversification, highlighting the open niches that remain to be explored by their bisexual relatives. 1. [↵][8]1. G. Bell , Masterpiece of Nature (Univ. of California Press, Berkeley, CA, 1982). 2. [↵][9]1. K. Wohrmann, 2. V. Loeschcke 1. R. C. Vrihenhoek , in Population Biology and Evolution, K. Wohrmann, V. Loeschcke, Eds. (Springer-Verlag, Berlin, 1984), pp. 217-231. [1]: /lookup/doi/10.1126/science.1069524 [2]: #ref-1 [3]: #xref-ref-1-1 "View reference 1 in text" [4]: {openurl}?query=rft.jtitle%253DAm.%2BMus.%2BNovit.%26rft.volume%253D3365%26rft.spage%253D1%26rft_id%253Dinfo%253Adoi%252F10.1206%252F0003-0082%25282002%2529365%253C0001%253APROWLO%253E2.0.CO%253B2%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [5]: /lookup/external-ref?access_num=10.1206/0003-0082(2002)365 2.0.CO;2&link_type=DOI [6]: #ref-2 [7]: #ref-3 [8]: #xref-ref-2-1 "View reference 1 in text" [9]: #xref-ref-3-1 "View reference 2 in text"