Origin of Asexual Reproduction in Hydra
Journal Title: Biomedical Journal of Scientific & Technical Research (BJSTR) - Year 2018, Vol 10, Issue 3
Abstract
Hydra's buds develop through the integrated activity of two different types of cells: a) Epithelial (aka epithelial-muscular) cells forming the didermic body wall (with extension into tentacles, hypostome, and foot) and b) Interstitial cells (aka amoeboid or basal cells) differentiating as cnidocytes (aka nematocytes, the cells that make cnidocysts, aka nematocysts), nerve, gland, and sex cells [1-3]. Over the years, I consolidated some ideas and data into a theory about how these different kinds of cells came to cooperate in budding [4-8]. I proposed that early in the Neoproterozoic Era a primitive two layered epithelial mat (resembling contemporary Placozoa [9-10]), was infected by amoeboid cells already equipped with an extrusion apparatus. The epithelia's attempt to reject the foreign cells failed but a symbiogenic relationship evolved and the duel system found a selective advantage in modified ejection. Ultimately, a mechanism for removing excess amoeboid cells was adapted for the production of buds. The adaptation hinged on three conditions: i. Amoeboid cells equipped with an extrusion apparatus were the ancestors of hydra's interstitial cells. ii. Hydra routinely produced excess cells that moved toward and accumulate in the budding region. iii. Excess cells form discrete modules that erupt as buds and are then "ejected." a) Amoeboid cells equipped with an extrusion apparatus were the ancestors of hydra’s interstitial cells. I am hardly the first to point to the presence of stinging apparatuses in protozoans and to similarities between protozoan "cnidocysts" and cnidarian cnidocysts: the "peduncle," "rhizoid," and "perforator" cnidocysts of dinoflagellates [11-14], the trichocysts of trypanosomes [15], zooflagellates [16] and mastigophorans [17], the "apicoplasts" (apical complexes) of "Sporozoa," the "polaroplast," of microsporidians [18], and the "polar capsules" of myxosporidians [19-25]. Jiri Lom, the distinguished Czech protozoologist and parasitologist, suggested that these "homologies [were] perhaps too close to be considered only a convergency phenomenon" [26], and Pierre Tardent, the renowned Swiss coelenterologist commented "The wheel didn't have to reinvent itself" [27], i.e., cnidrians didn’t have to invent cnidocysts. It is a small step to extend Lynn Margulis' hypothesis of endosymbiosis [28] from mitochondria and chloroplasts to cnidocysts. Ancient eukaryotic amoeba could have been infected by monerans (presumably bacteria) already equipped with an eversion apparatus. The "guest” would then introduce genes to the "host" through unilateral horizontal gene transfer, and a permanent extrusion apparatus (a cnidocyst) would have evolved in the amoeba. What followed was a different sort of symbiosis -symbiogeny - the merging and mutual evolution of eukaryotes [5-6]. In the case of cnidarians, at some point in the evolution of multicellular eukaryotic life, probably prior to the Vendian Period 700 million years ago, an amoeboid protoctistan (to use Margulis' term [29], aka protozoan) already equipped with an extrusion apparatus gained access to a primitive didermic metazoan mat either by invading or being ingested. I suggest that, at the time, mechanisms for isolating different forms of metazoan life were not as restrictive as they became since and the efforts of the epithelia to reject the amoeba were feeble.
Authors and Affiliations
Stanley Shostak
Keywords
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- EP ID EP588396
- DOI 10.26717/BJSTR.2018.10.001966
- Views 151
- Downloads 0
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