Chapter

Hox-genes in the ontogenesis of polychaetes

Milana a. Kulakova*

Department of Embriology

Saint Petersburg University

Saint Petersburg

Russia

Abstract

The basic plane and evolution of bilateral animals (Bilateria) are closely associated with Hox-genes. In the general case these genes exist in the genome in the form of clusters - complexes of genes with a conservative position of individual genes inside. These genes code transcription factors, containing homeodomain and their expression is ordered along a main body axis in accordance with a colinearity rule. This means that position of gene in cluster determines his spatial and sometimes temporary expression in embryo. In vertebrates and arthropods, which are investigated most detailed, Hox genes define a morphological distinction between different regions of body (tagmas). Boundaries of Hox genes expression are often coincide with borders of tagmas. In addition, these genes also have function in adult organisms, where they are involved in regeneration processes and maintain a tissue homeostasis. In recent years, there was an emergence of interest in the study of Hox genes in animals that do not belong to the traditional models of molecular biology (Drosophila, mouse, Xenopus). The Polychaetes belong to the most intriguing objects for such studies. These are members of the third, poorly studied evolutionary branch of Bilateria - Lophotrochozoa, which itself makes them attractive both for molecular biologists and developmental biologists. Among of the polychaetes, there are groups (families) with morphologically specialized segments, which are grouped in the tagmas (Chaetopteridae) and families with a large number of identical segments (Nereididae). There are species with impressive abilities to regeneration of the head and tail and those that are incapable of regeneration at all. In addition, often a larva exists within the life cycle of polychaetes. The larva may be radically different from an adult worm. How work the Hox genes in such different systems? Can we compare some of their functions with the functions of homologous genes in insects and vertebrates? What is their place in the hierarchy of molecular regulators, which controlling developmental processes in the polychaets? These questions arose not because of idle scientific curiosity. If we can correctly answer them, many pieces of bilaterian animals earliest evolution jigsaw puzzle will fall into place. In this review, I attempted to collect all the modern data on Hox genes in polychaetes. Also there was made some cautious assumptions about the ancestral functions of the Hox cluster.

Keywords: Annelida, Hox genes, UrBilateria, Larval development, Regeneration.

1. Introduction.

This chapter deals with Hox-genes, their structural organization and function features in polychaetes ontogenesis. Within hundreds of evolutionary conservative gene families Hox-genes are particularly important for understanding the relation between individual development and the macroevolution. Edward B. Lewis, the American genetic scientist (Nobel Prize in Medicine 1995) was first who discovered this relation. In 1987 he published in Nature the results of long-term study about segmentation control pattern in Drosophila (Lewis, 1987). Lewis used mutation analysis to consider the structure and the function of specific bithorax locus (a gene complex bithorax or BX-C). As early as 1915 Calvin Bridges (who worked in Thomas Hunt Morgan’s laboratory) isolated and described strange bithorax mutation at this locus in Drosophila. This was a classic homeotic mutation in which third thoracal segment was partially transformed into the second one. In this case the anterior part of haltere was turned into the anterior part of the wing (Bridges and Morgan, 1923).

Lewis knew nothing about nature of factors which are encoded in sequence BX-C, but he made some important conclusions. Most of them proved true brilliantly. There are some of them:

1. BX-C genes form a cluster – the complex of genes, which emerged as a result of tandem duplications of ancestor gene with subsequent mutational divergence of its offsprings;

2. There is direct link of gene’s position at the chromosome with spatial order of gene’s activation (now we call this link by the term “colinearity”);

3. Diptera originated from primitive four-winged ancestors, and insects originated from primitive arthropods, which had legs at all abdominal segments. During the evolution “leg-suppressing” genes emerged, which suppressed legs development at abdominal segments. Also emerged “haltere-promoting” genes which suppressed wing development at the third thoracal segment. The loss-of-function mutation in BX-C may lead to primitive state recapitulation, i.e. to the appearance of four-winged and many-legged flies.

Now we know that in the insect’s evolution there wasn’t an emerging of new Hox-genes, instead “old” genes assumed new development program control functions. However, the logical mainstream used by Lewis, was true.

Just after the publication of this key article, researchers had obtained the tool, which directly linked the molecular control of development and the phylogenetic evolution. There was born the new science – the Evolutionary Developmental biology or EvoDevo.

The search, the cloning and the sequencing of BX-C sequences and of other Drosophila genes (which are causing homeotic transformation) was performed independently by three scientist groups in California, Indiana and Basel (cit. ex Papageorgiou, 2007). As Lewis had predicted, these genes appeared to be serial homologs. It became known that they encode transcription factors with distinctive DNA-binding protein domain – the homeodomain. The nucleotide fragment that encoding this homeodomain, consists of 180 base pairs and is named homeobox. Hox-genes (from words “homeotic” and “homeobox”) in Drosophila are organized in two complexes: ANT-C and BX-C. Later a set of homologous Hox-genes was found in Vertebrates (Ranginib et al., 1989; Godsave et al., 1994; Burke et al., 1995). The mutations of these genes also led to spatial shifts in structures that located along body axis. It suggested that Hox-genes had originated not in insects. Their evolutionary age appeared to be much older. Hox-genes originated in the remote past, deep in Precambrian time. It was about 600 millions years ago (mya), when supposedly lived the last common ancestor of chordates and arthropods.

There was analysis of Hox amino-acid sequences of homeodomains and flanking areas in animals from different evolutionary branches. Its results dramatically changed the phylogeny of bilateral animals (de Rosa et al., 1999). It appears that evolutionary tree of bilateral animals is divided near the basement to two stems: Deuterostomia (Chordates, Hemichordates, Echinoderms) and Protostomia. The last ones in turn are divided to Ecdysozoa (Arthropods, Onychophorans, Nematodes, Priapulida and others) and Spiralia (Brachiopods, Nemerteans, Annelids, Mollusca, Platyhelminthes, Rotifers and others) (de Rosa et al., 1999). The term “Ecdysozoa” derives from Greek word “ecdysis” for molting, because all Ecdysozoa animals are molting ones, covered by hard cuticle. Their growth is accompanied by periodical molting, when old covers are dropped and the animal is growing quickly (more truly straighten itself up until new cover harden). The Spiralia branch named after common stereotype of early embryonic cleavage. This branch is most heterogeneous and interesting. It include Lophotrochozoa, Platyhelminthes, Gastrotricha, Syndermata and Gnathostomulida (Struck et al., 2014). The Lophotrochozoa group especially intrigues evolutionary biologists, because there is highest divergence of body organization plans. Indeed octopus, murex and ostrea are representatives of same taxon Mollusca, but it is hard to imagine by looking at their appearance. The term Lophotrochozoa itself derives from two other zoological terms: Lophophorata and Trochophora. Lophophorata (Brachiopoda, Bryozoa and Phoronida) are animals with lophophore – the especial organ for feed by particles which are suspended in water. Earlier Lophophorata was included in Deuterostomia. The Trochophora is ciliary spherical larva, tipically occurring in Lophotrochozoa (Annelids, Mollusca).

This new “molecular” phylogeny had fitted with previously built phylogeny by 18s rDNA (Field et al., 1988, Aguinaldo et al., 1997) and by mitochondrial DNA (Cohen et al., 1998). The comparison study of Hox-genes sequences suggested that common ancestor of bilateral animals (located at the basement of all three evolutionary branches) already had had at least seven Hox-genes (de Rosa et al., 1999). How looked this common ancestor, what processes had led to its appearance and what is the cause of great evolutionary radiation of its descendants – these are the questions which consist a paradigm of modern science about Metazoa evolution.

During last ten years, mechanisms underlying morphological evolution became mostly clear. New outstanding methods emerged, which allow to delicately tune the work of gene of interest, to turn off this gene in suitable time of development, to stimulate its ectopic expression and even to edit a genome in vivo (CRISPR/Cas9 technology). However our knowledge about Hox-gene’s functions and even about patterns of their expression is unequal for different Bilateria branches. While Hox-gene functions within Deuterostomia and Ecdysozoa have been studied in detail at least et the vertebrates and arthropods examples, in contrast these researches of Lophotrochozoa branch are at the beginning stage. Meanwhile the set of molecular tools for control over morphogenesis of these animals is same as in Drosophila or in human. The difference is in strategy of these tools using. If we pay more attention to Lophotrochozoa group, we’ll meet wonderful discoveries in regulatory evolution and epigenetics.