Spiralian model systems

Abstract

The "Spiralia" represent one of the three major clades of bilaterian metazoans. Though members of this clade exhibit tremendous diversity in terms of their larval and adult body plans, many share a highly conserved early pattern of development involving a stereotypic cleavage program referred to as spiral cleavage. This group therefore represents an excellent one in which to undertake comparative studies to understand the origins of such diversity from a seemingly common ground plan. These organisms also present varied and diverse modes in terms of their ecology, development and life history strategies. A number of well established and emerging model systems have been developed to undertake studies at the molecular, genetic, cell and organismal levels. The Special Issue of the Int. J. Dev. Biol. entitled "Spiralian Model Systems" focuses on these organisms and here, I introduce this clade, pointing out different types of studies being undertaken with representative spiralian model systems. KEY WORDS: bilaterian metazoan, spiral cleavage, life history strategyThe Spiralia (Lophotrochozoa)Of the three major clades of bilaterians, the Spiralia (Lophotrochozoa) comprise nearly half of the extant metazoan phyla (see Fig. 1). Despite this fact, the group has received relatively little attention compared to the other two clades, the deuterostomes and ecdysozoans, notably in the areas of genetics, as well as molecular, cellular and developmental biology. This is due in part to the long standing predominance of key experimental models positioned within the Ecdysozoa, (e.g., the fruit fly Drosophila and the nematode, C. elegans), and the Deuterostomia, (e.g., chordates such as the Zebrafish and mouse, as well as a few invertebrate representatives from the Echinodermata).The Spiralia include 14 of roughly 36 metazoan phyla (Fig. 1). The Spiralia include the Lophotrochozoa, and sometimes these terms have been used synonomously. The clade "Lophotrochozoa" was first recognized by Halanych et al., (1995, see also Giribet et al., 2007 Helmkampf et al., 2008a,b;Dunn et al., 2008;Hejnol et al., 2009; Edgecomb et al., 2011) who showed that the Lophophorata (consisting of groups possessing characteristic ciliated feeding structures, such as brachiopods and phoronids), are clearly united with other protostome phyla that include annelids, molluscs, and nemerteans. The Spiralia, however, encompass an even larger group of metazoans, and the exact relationships amongst the Spiralia are, however, not fully resolved. The consensus from recent analyses suggest that there are two large sub-groups (clades). One group is the "Trochozoa," (Roule, 1891), which include Annelida, Int. J. Dev. Biol. 58: 389-401 (2014) doi: 10.1387/ijdb.140127jh Mollusca, Nemertea, as the "Eutrochozoa", together with the "Brachiozoa" (see Cavalier-Smith, 1998), comprised of Brachiopoda and Phoronida, and the "Polyzoa" (Funch and Kristensen 1995;Passamaneck and Halanych 2006; Helmkampf et al., 2008a,b;Edgecombe et al., 2011) consisting of the Bryozoa, E...

Deployment of regulatory genes during gastrulation and germ layer specification in a model spiralian mollusc Crepidula

“…Many members of the Spiralia exhibit a highly conserved spiral cleavage pattern and cell lineage fate map (Hejnol, ; Lambert, ; Henry, ). This characteristic spiral cleavage program, together with molecular phylogenetic analyses, reveals the close evolutionary relationships amongst members of the Spiralia (Boyer et al, ; Henry and Martindale, ; Dunn et al, ; Giribet, ; Hejnol, ; Lambert, ; Henry, ). Thus understanding germ layer specification in the Spiralia, particularly in those with spiral cleavage, which is believed to be the ancestral condition for this clade, should expand our understanding of how this clade evolved.…”

“…In contrast, germ layer specification is not well understood in the Spiralia, a major branch of bilaterians (Boyle et al, 2014;Passamaneck et al, 2015). The Spiralia contains animals such as annelids, molluscs, brachiopods, phoronids, rotifers, among others, with diverse larval and adult body plans (Hejnol, 2010;Henry, 2014). Many members of the Spiralia exhibit a highly conserved spiral cleavage pattern and cell lineage fate map (Hejnol, 2010;Lambert, 2010;Henry, 2014).…”

Ectomesoderm and epithelial–mesenchymal transition‐related genes in spiralian development

“…The Spiralia (including Lophotrochozoa) represent the largest clade of bilaterian metazoans, which is comprised of fourteen different phyla (such as Annelida, Mollusca, Nemertea, Platyhelminthes, etc. ; Struck et al, ; Henry, ). A conserved feature of this clade, and the one from which its name is derived, is the highly stereotyped embryonic cleavage pattern, referred to as “spiral cleavage.” This cleavage pattern is found in members from at least seven of those phyla (Fig.…”

“…Like most metazoans, spiralians are characterized by the presence of triploblastic body plans, wherein processes of early cleavage and gastrulation generate three principal germ layers (ectoderm, mesoderm, and endoderm). Such studies have shown that mesoderm arises from two different sources in spiralian embryos (reviewed by Henry, ). One source is the “endomesoderm,” which is typically derived from the dorsal D quadrant.…”

A set of simple cell processes are sufficient to model spiral cleavage

“…if the third division produces a micromere to the left). Owing to this alternation, the four new micromeres appearing between the 4-and 8-cell stages seem to twist clockwise (in the dextral pattern) or counterclockwise (in the sinistral pattern) when viewed from the animal pole (with respect to the four underlying macromeres) (Gilbert and Raunio, 1997;Henry, 2014). Compared with those of other groups, spiralian embryos tend to undergo fewer cell divisions before gastrulation, making it easier to follow the fate of their blastomeres (Gilbert and Raunio, 1997).…”