Inclusive Taxon Sampling Suggests a Single, Stepwise Origin of Ectolecithality in Platyhelminthes
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bs_bs_banner Biological Journal of the Linnean Society, 2014, 111, 570–588. With 3 figures Inclusive taxon sampling suggests a single, stepwise origin of ectolecithality in Platyhelminthes CHRISTOPHER E. LAUMER* and GONZALO GIRIBET FLS Downloaded from https://academic.oup.com/biolinnean/article/111/3/570/2415786 by guest on 30 September 2021 Museum of Comparative Zoology & Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA Received 16 September 2013; revised 7 November 2013; accepted for publication 11 November 2013 Ectolecithality is a form of oogenesis unique within Metazoa but common in Platyhelminthes, in which almost yolkless oocytes and tightly associated yolk cells are deposited together in egg capsules. Despite profound impacts on the embryogenesis and morphology of its beneficiaries, the origins of this developmental phenomenon remain obscure. Traditionally, all ectolecithal flatworms were grouped in a clade called Neoophora. However, there are also morphological arguments for multiple origins of ectolecithality and, to date, Neoophora has seen little support from molecular phylogenetic research, largely as a result of gaps in taxon sampling. Accordingly, we present a molecular phylogeny focused on resolving the deepest divergences among the free-living Platyhelminthes. Species were chosen to completely span the diversity of all major endo- and ectolecithal clades, including several aberrant species of uncertain systematic affinity and, additionally, a thorough sampling of the ‘lecithoepitheliate’ higher taxa Prorhynchida and Gnosonesimida, respectively, under- and unrepresented in phylogenies to date. Our analyses validate the monophyly of all classical higher platyhelminth taxa, and also resolve a clade possessing distinct yolk-cell and oocyte generating organs (which we name Euneoophora new taxon). Furthermore, implied-weights parsimony and Bayesian mixture model analyses suggest common ancestry of this clade with the lecithoepitheliates, implying that these taxa may retain a primitive form of ectolecithality. This topology thus corroborates the classical hypothesis of homology between yolk cells and oocytes in all Neoophora, and should serve to guide future evolutionary research on this unique developmental innovation in Platyhelminthes. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 570–588. ADDITIONAL KEYWORDS: phylogenetics – Neoophora – Gnosonesimida – Prorhynchida – mixture model – heterotachy – implied weights parsimony. INTRODUCTION et al., 1991), a hypothesis that has seen no recent support in light of recent molecular phylogenies Flatworms (phylum Platyhelminthes) are among (Baguñà & Riutort, 2004), the most speciose flat- Earth’s dominant invertebrate animals, with over worm groups (e.g. Tricladida, Rhabdocoela, and 6500 described free-living (Tyler et al., 2012) and Neodermata) have diverged remarkably from the ana- 20 000 parasitic species (Caira & Littlewood, 2013). tomical and developmental simplicity commonly Indeed, their total diversity may be over an order of evoked in discussions of this phylum. In particular, as magnitude larger as a result of their predominance hermaphrodites bestowed with diverse mechanisms as members of the marine meiofauna (Martens & for achieving internal fertilization, platyhelminths Schockaert, 1986) and as parasites of vertebrate and have developed some of the most elaborate reproduc- invertebrate hosts (Poulin & Morand, 2000), two of tive systems among Metazoa (Rieger et al., 1991; the least-known reservoirs of global biodiversity. Conn, 2000). Although historically considered to retain characters A key component of this reproductive complexity is predating the diversification of Bilateria (Rieger the existence in most flatworm species and higher taxa of a condition known as ectolecithality, the *Corresponding author. E-mail: [email protected] spatial partitioning of the products of oogenesis into 570 © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 570–588 ECTOLECITHALITY IN PLATYHELMINTHES 571 (C) Downloaded from https://academic.oup.com/biolinnean/article/111/3/570/2415786 by guest on 30 September 2021 (D) (E) (A) (B) Figure 1. Proposed phylogenetic relationships between endolecithal, lecithoepitheliate, and euneoophoran ectole- cithal higher platyhelminth taxa (branches coloured as in Fig. 3). A, summary of Bayesian mixture model analyses of Matrix B. Nodal support values below 1 are given as posterior probabilities from a reversible-jump Markov chain Monte Carlo run for 5.747 × 107 generations, with the first 25% discarded as burn-in. Olive dots represent branches for which two lengths were fit to model heterotachy, with posterior probability > 0.9. B, summary of implied weights parsimony (k = 3) analyses of Matrix B. Nodal support values below 100 are given as the proportion of 100 symmetric resampling replicates supporting each node. C, D, E, schematic illustrations of the female reproductive system of (C) an endolecithal, (D) a gnosonesimid lecithoepitheliate, abstracted into unipartite form, and (E) a euneoophoran platyhelminth, oriented with germinal areas to the left, and the female gonopore to the right. ge, germarium (ovary); gv, germovitellarium (compound ovary/yolk gland); oc, oocyte; sg, shell glands; ut, uterus; vc, vitellocyte (yolk cell); vt, vitellarium (yolk gland) almost or entirely yolkless oocytes and separate, embryonic development. During oviposition, ectoleci- specialized yolk-bearing cells (sometimes called thal flatworms deposit hundreds of mature vitello- vitellocytes; Gremigni, 1983) (Fig. 1D, E). Indeed, cytes into thick-shelled egg capsules alongside one or only three platyhelminth higher taxa (Catenulida, several fertilized zygotes (Shinn, 1993). Development Macrostomorpha, and Polycladida) retain a plesio- then proceeds with the small, almost yolkless embryo morphic (endolecithal) condition, producing a single surrounded by these yolk cells. Although at least major cell type during oogenesis, as in most Metazoa polyclads progress through a recognizable spiralian (Fig. 1C) ( Rieger et al., 1991). Ectolecithality is also a cleavage program (Surface, 1907; Boyer, Henry & functional division: in the species that possess them, Martindale, 1998; Rawlinson, 2010), early develop- vitellocytes are largely or entirely responsible for the ment in many other flatworm taxa shows little simi- synthesis and storage of yolk and the formation of a larity to other metazoans (Martín-Durán & Egger, protective shell by marginal granules and shell gland 2012). In ectolecithal flatworms in particular, such secretions (Gremigni, 1988; Swiderski & Xylander, developmental modifications may be especially pro- 2000), which are tasks usually accomplished by the found: most taxa form a ‘hull membrane’, a transient oocyte and/or its accessory cells in other metazoa structure used to envelop and secure vitellocytes (Shinn, 1993). within or against the developing embryo, and many In further contrast to the accessory cell types groups (including planarian model systems; Sánchez (e.g. nurse cells or follicle cells) known from other Alvarado, 2003) display no trace of spiral cleavage; metazoa, flatworm vitellocytes play an active role in instead, they undergo a dispersive cleavage and ‘blas- © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 570–588 572 C. E. LAUMER and G. GIRIBET tomere anarchy’ in which blastomeres ephemerally F; Timoshkin, 1991a), and Gnosonesimida, a rare lose physical contact and develop adrift in a matrix of meiobenthic group of six species presently classified external, syncytial yolk. Although almost nothing is in the single genus Gnosonesima, known from shallow known about the developmental mechanisms under- (intertidal to 350 m) marine sediments (Fig. 2D) lying these processes (in large part because of diffi- (Karling, 1968). The sole hypothesized synapomorphy culties in observing and manipulating ectolecithal of these taxa is the structure of their female gonads, embryos within their capsules), these modifications which are organized as chains of follicles, each con- have been explained in evolutionary terms as embry- sisting of a layer of vitellocytes more or less tightly onic adaptations to the presence of external yolk enveloping single oocytes, thus organized as the (Thomas, 1986; Martín-Durán & Egger, 2012). consequence of a common germinative zone for Ectolecithality is also associated with increased func- vitellocytes and oocytes in this taxon (Fig. 1D) (Rieger Downloaded from https://academic.oup.com/biolinnean/article/111/3/570/2415786 by guest on 30 September 2021 tional compartmentalization in the adult reproductive et al., 1991). Historically, the tetrapartite germovi- morphology of most taxa: among the major lineage of tellaria of Gnosonesima was proposed to represent parasitic Platyhelminthes (Neodermata) in particular, the primitive condition from which other ectolecithal the presence of ectolecithality has allowed a spectacu- taxa diverged (Reisinger, 1926); later systematists lar anatomical specialization of female reproductive have followed this convention, placing a monophy- organs (exemplified by the compound assembly of letic Lecithoepitheliata as the sister group to the an egg capsule in the digenean ootype; Ehlers, 1985; remaining Neoophora, a hypothesis that persists Coil, Harrison & Bogitsh, 1991; Fried & Haseeb, to the present (Martín-Durán