Embryology of the Turbellaria and its phylogenetic significance

Abstract

Developmental characters -including oocyte and yolk cell structure, patterns of cleavage, and modes of gastrulation -are presented and examined in relation to the phylogeny of the Turbellaria . Eggshell granules, which have been demonstrated to occur in the oocytes of entolecithal eggs and the yolk cells of ectolecithal eggs, are compared among species, and their potential value as a taxonomic character is discussed . The quartet 4d spiral cleavage of the entolecithal egg of polyclads is described as reminiscent of the primitive pattern of early development for the Tlrbellaria . This is compared to duet spiral cleavage of acoels, and possible phylogenetic schemes involving the two types of spiral cleavage are reviewed . The link between the precise spiral cleavage, which characterizes development of most archoophorans, and blastomere separation (Blastomeren Anarchie), which occurs in several neoophoran orders, is established by the occurrence of quartet 4d spiral cleavage in one neoophoran order, and of both quartet spiral cleavage and Blastomeren Anarchie in different species of a second neoophoran order . The epibolic gastrulation of polyclads is described as primitive for the Turbellaria because of its similarity to that of other members of the Spiralia . Although no identical process occurs in neoophoran development, the earlier event of formation of the hull membrane in some neoophorans, and the later event of formation of the definitive epidermis in all neoophorans studied are presented as processes of possible homology to the epibolic gastrulation of polyclads . The lack of correspondence between polyclads and neoophorans in the relationship of the definitive body axes to the egg axis is discussed, and an hypothesis is advanced to account for the differences . The phylogenetic relationships indicated by known developmental phenomena differ only slightly from the scheme presented by Karling in 1974. Hydrobiologia 132, 105-115 (1986).

Developmental diversity in free-living flatworms

“…Some authors have applied the concept of phylotypic stage to the study of flatworm embryology [75]. Based on their descriptions of the embryonic development of polyclads and rhabdocoels [45,67,74], stage 6 (according to the staging proposed by [67]) was suggested as the moment in development when the basic features of the body plan of a (rhabditophoran) platyhelminth are defined, namely a ventral definitive epidermis, an anterior neuropile with incipient ventral nerve cords, a ventral pharynx and developing muscle layers, and a pair of lateral bands with nephridial cells and myoblasts, which closely corresponds to the types proposed by earlier authors [3,30]. The most recent studies of the embryonic development of triclads support considering stage 6 as the point in development when the basic body plan of a triclad is defined, as in other platyhelminths [89,96,97].…”

“…The latter process bears similarity to the epibolic gastrulation in polyclads [30,71]. Some authors have applied the concept of phylotypic stage to the study of flatworm embryology [75].…”

The embryonic development of the rhabdocoel flatworm Mesostoma lingua (Abildgaard, 1789)

“…For example, in the polyclads, the micromere 4d produces both endoderm and mesoderm. In flatworms, micromeres 4a-c are the main yolk-containing cells that are incorporated into the gut, and macromeres degenerate (Surface 1908;Kato 1940;Thomas 1986;Boyer et al 1996). This differs from the spiral archetype (e.g., mollusks and annelids), where 4d makes only mesoderm, and 4a-c and the macromeres endoderm.…”

“…The so-called "archoophorans", which include groups considered to be more ancestral in most phylogenetic schemes (e.g., macrostomids, polyclads), produce yolk-rich oocytes that undergo a somewhat modified form of spiral cleavage (for more details, see Discussion). Gastrulation proceeds by epiboly of the animal layer of presumptive ectoderm cells over the vegetal, yolk-laden endoderm and mesoderm progenitors (Thomas 1986). The platyhelminth groups that are more highly evolved phylogenetically and are included among the neoophorans (e.g., triclads, proseriates, lecithoepitheliates, prolecithophorans, typhloplanoids) have small oocytes, invested by a multilayer of yolk cells formed by specializations of the ovaries (vitellarium).…”

“…In more recent times, few general descriptions of flatworm embryogenesis have been added, with the notable exceptions of Giesa's (1966) description of the proseriate Monocelis, Apelt's (1969) study on acoels, the study by Reisinger et al (1974a, b) on lecithoepitheliates and proseriates, Newton's (1970) study of the prolecithophoran Hydrolimax, and the works by Bennazzi and Gremigni (1982) on triclads. The review articles by Thomas (1986) and Baguna and Boyer (1990) provide a helpful overview of the different modes of embryogenesis of the platyhelminth taxa in relation to their phylogenetic position. Considerable ultrastructural work has been done in regard to specific organ systems, for example epidermis (e.g., Tyler 1981;reviewed in Ehlers 1985;Tyler and Tyler 1997), body wall musculature (Reiter et al 1996), and eye (Lanfranchi et al 1986).…”