Conservation, loss, and redeployment of Wnt ligands in protostomes: implications for understanding the evolution of segment formation

Abstract

BackgroundThe Wnt genes encode secreted glycoprotein ligands that regulate a wide range of developmental processes, including axis elongation and segmentation. There are thirteen subfamilies of Wnt genes in metazoans and this gene diversity appeared early in animal evolution. The loss of Wnt subfamilies appears to be common in insects, but little is known about the Wnt repertoire in other arthropods, and moreover the expression and function of these genes have only been investigated in a few protostomes outside the relatively Wnt-poor model species Drosophila melanogaster and Caenorhabditis elegans. To investigate the evolution of this important gene family more broadly in protostomes, we surveyed the Wnt gene diversity in the crustacean Daphnia pulex, the chelicerates Ixodes scapularis and Achaearanea tepidariorum, the myriapod Glomeris marginata and the annelid Platynereis dumerilii. We also characterised Wnt gene expression in the latter three species, and further investigated expression of these genes in the beetle Tribolium castaneum.ResultsWe found that Daphnia and Platynereis both contain twelve Wnt subfamilies demonstrating that the common ancestors of arthropods, ecdysozoans and protostomes possessed all members of all Wnt subfamilies except Wnt3. Furthermore, although there is striking loss of Wnt genes in insects, other arthropods have maintained greater Wnt gene diversity. The expression of many Wnt genes overlap in segmentally reiterated patterns and in the segment addition zone, and while these patterns can be relatively conserved among arthropods and the annelid, there have also been changes in the expression of some Wnt genes in the course of protostome evolution. Nevertheless, our results strongly support the parasegment as the primary segmental unit in arthropods, and suggest further similarities between segmental and parasegmental regulation by Wnt genes in annelids and arthropods respectively.ConclusionsDespite frequent losses of Wnt gene subfamilies in lineages such as insects, nematodes and leeches, most protostomes have probably maintained much of their ancestral repertoire of twelve Wnt genes. The maintenance of a large set of these ligands could be in part due to their combinatorial activity in various tissues rather than functional redundancy. The activity of such Wnt 'landscapes' as opposed to the function of individual ligands could explain the patterns of conservation and redeployment of these genes in important developmental processes across metazoans. This requires further analysis of the expression and function of these genes in a wider range of taxa.

Involvement of the Wnt/β-catenin pathway in neurectoderm architecture in Platynereis dumerilii

“…This is similar to what has been observed in vertebrates, in which axin2 is a transcriptional target of the Wnt/b-catenin pathway in many tissues 33 , and suggests that a negative feedback loop involving axin participates in the regulation of the Wnt/b-catenin pathway activity in the Platynereis neurectoderm. Given that Pdu-wnt4 is the only Platynereis wnt gene expressed by ventral midline cells 20 and that Pdu-axin is only expressed in the cells that are located very close to the ventral midline, we suggest that a Wnt4 signal emanating from the ventral midline cells would be responsible for the activation of Pdu-axin expression. This assumption is reinforced by the expression in midline cells of Pdu-Wntless/Evi whose orthologues are required for Wnt protein secretion 21,22 .…”

The Evolution of the Wnt Pathway

“…Other annelids and mollusks have lost several Wnt gene subfamilies, that is, Wnt8, -9, and -A in Helobdella robusta or Wnt4, -5, -6, -7, -8, -9, -11, and -16 in Patella vulgata (Prud'homme et al 2002;Cho et al 2010;Janssen et al 2010;Riddiford and Olson 2011). The Wnt genes of P. dumerilii show blastoporal and posterior expression in the growth zone (segment addition zone) of trochophoran larvae, and at the posterior side of each segment (Janssen et al 2010), reminiscent to arthropods (see below).…”

“…The other major ecdysozoan groups are arthropods. Basal arthropods show a rich Wnt gene repertoire, indicating that the common ancestor of arthropods only lacked Wnt-3 (Janssen et al 2010). Members of 12 Wnt gene subfamilies were characterized at the transcriptional level (Janssen et al 2010).…”

“…Basal arthropods show a rich Wnt gene repertoire, indicating that the common ancestor of arthropods only lacked Wnt-3 (Janssen et al 2010). Members of 12 Wnt gene subfamilies were characterized at the transcriptional level (Janssen et al 2010). A relatively complete set of protostome Wnt genes was found in the genomes of the crustacean Daphnia pulex (12 Wnt subfamilies), and in chelicerates and myriapods, where Wnt10 has been lost (Janssen et al 2010).…”

-Catenin-Dependent Wnt Signaling in C. elegans: Teaching an Old Dog a New Trick

“…The C. elegans genome contains only five genes encoding Wnt ligands: lin-44, egl-20, mom-2, cwn-1, and cwn-2 (Shackleford et al 1993;Herman et al 1995;Rocheleau et al 1997;Thorpe et al 1997;Maloof et al 1999), a lower number than in other animals. It may have once been assumed this small number was due to some "lower" evolutionary status of nematodes, but genome sequencing of diverse animal species now suggests that the early metazoan common ancestor had a large complement of Wnt genes and that nematode lineages have lost many of these genes (Guder et al 2006;van Amerongen and Nusse 2009;Janssen et al 2010;Pang et al 2010). Surprisingly, only one of the C. elegans Wnts, MOM-2, appears to play a major role during embryogenesis: 70% of mom-2 zygotic mutant animals die during embryogenesis, whereas ,5% of lin-44; cwn-1; cwn-2 or lin-44; cwn-1; egl-20 mutants display embryonic lethality (Thorpe et al 1997;Gleason et al 2006).…”