Michael Akam

Mechanisms of germ cell specification across the metazoans: epigenesis and preformation.

Germ cells play a unique role in gamete production, heredity and evolution. Therefore, to understand the mechanisms that specify germ cells is a central challenge in developmental and evolutionary biology. Data from model organisms show that germ cells can be specified either by maternally inherited determinants (preformation) or by inductive signals (epigenesis). Here we review existing data on 28 metazoan phyla, which indicate that although preformation is seen in most model organisms, it is actually the less prevalent mode of germ cell specification, and that epigenetic germ cell specification may be ancestral to the Metazoa.

Hox genes in brachiopods and priapulids and protostome evolution

Understanding the early evolution of animal body plans requires knowledge both of metazoan phylogeny and of the genetic and developmental changes involved in the emergence of particular forms. Recent 18S ribosomal RNA phylogenies suggest a three-branched tree for the Bilateria comprising the deuterostomes and two great protostome clades, the lophotrochozoans and ecdysozoans. Here, we show that the complement of Hox genes in critical protostome phyla reflects these phylogenetic relationships and reveals the early evolution of developmental regulatory potential in bilaterians. We have identified Hox genes that are shared by subsets of protostome phyla. These include a diverged pair of posterior (Abdominal-B -like) genes in both a brachiopod and a polychaete annelid, which supports the lophotrochozoan assemblage, and a distinct posterior Hox gene shared by a priapulid, a nematode and the arthropods, which supports the ecdysozoan clade. The ancestors of each of these two major protostome lineages had a minimum of eight to ten Hox genes. The major period of Hox gene expansion and diversification thus occurred before the radiation of each of the three great bilaterian clades.

Hox and HOM: Homologous gene clusters in insects and vertebrates

Michael Akam Department of Genetics University of Cambridge Cambridge CB2 3EH England In Drosophila, homeotic genes of the Antennapedia (ANT- C) and Bithorax (BX-C) complexes specify the distinctions between different segments of the body. Each of these genes contains a conserved DNA sequence, the homeo- box, that encodes a DNA binding homeodomain in the protein. When the homeobox was first identified, it was hailed as a motif unique to segmentation genes (Gehring, 1987). It is now clear that homeoboxes are present in a wide range of eukaryotic regulatory genes. Yet results on the detailed structure, organization, and expression of An- tennapedia (An@)-like genes in vertebrates (Gaunt et al., 1988; Duboule and Dolle, 1989; Graham et al., 1989) sug- gest that the vertebrate Hox clusters containing these genes are true homologs of the insect homeotic gene complexes (HOM-C). It appears that both HOM-C and Hox contain, in the same chromosomal order, the descendants of a gene family whose main members were already dis- tinct when insect and vertebrate lineages diverged. Moreover, corresponding genes of the vertebrate and in- sect complexes show the same relative boundaries of ex- pression along the antero-posterior (A-P) axis of the de- veloping embryo. It is hard to escape the conclusion that insects and vertebrates have inherited from a common an- cestor a conserved molecular representation of front, mid- dle, and back. The Antp homeodomain adopts in solution a conforma- tion closely similar to the helix-turn-helix structure of bac- terial repressor proteins (Otting et al., 1988). Limited se- quence similarity underlies this common structure, but these critical residues are encoded by virtually all homeo- boxes, and are also seen in the yeast mating type proteins (Scott et al., 1989), suggesting that all these proteins uti- lize variants of the same DNA binding domain. The first homeoboxes to be identified were detected by DNA hybridization with Antp-like probes, and so of neces- sity were relatively similar in sequence to An@. As more divergent homeoboxes are detected by direct comparison of sequence data, it is becoming harder to define pre- cisely what is and what is not a homeobox. At the end of the day (and particularly when higher plants have been adequately screened), the family of homeodomain pro- teins may merge indistinguishably with other helix-turn- helix proteins that show no more homology to Antp than they do to the yeast mating type proteins. In Drosophila the most divergent members of this family have no func- tional association with segmentation, and in vertebrates they include both general and cell-type-specific transcrip- tion factors. While the homeodomain may characterize

Arthropod Segmentation: beyond the Drosophila paradigm

Most of our knowledge about the mechanisms of segmentation in arthropods comes from work on Drosophila melanogaster. In recent years it has become clear that this mechanism is far from universal, and different arthropod groups have distinct modes of segmentation that operate through divergent genetic mechanisms. We review recent data from a range of arthropods, identifying which features of the D. melanogaster segmentation cascade are present in the different groups, and discuss the evolutionary implications of their conserved and divergent aspects. A model is emerging, although slowly, for the way that arthropod segmentation mechanisms have evolved.

Hox genes and the phylogeny of the arthropods

The arthropods are the most speciose, and among the most morphologically diverse, of the animal phyla. Their evolution has been the subject of intense research for well over a century, yet the relationships among the four extant arthropod subphyla - chelicerates, crustaceans, hexapods, and myriapods - are still not fully resolved. Morphological taxonomies have often placed hexapods and myriapods together (the Atelocerata) [1, 2], but recent molecular studies have generally supported a hexapod/crustacean clade [2-9]. A cluster of regulatory genes, the Hox genes, control segment identity in arthropods, and comparisons of the sequences and functions of Hox genes can reveal evolutionary relationships [10]. We used Hox gene sequences from a range of arthropod taxa, including new data from a basal hexapod and a myriapod, to estimate a phylogeny of the arthropods. Our data support the hypothesis that insects and crustaceans form a single clade within the arthropods to the exclusion of myriapods. They also suggest that myriapods are more closely allied to the chelicerates than to this insect/crustacean clade.

Drosophila: the genetics of two major larval proteins

A series of irradiation-induced deficiencies covering 62 polytene chromosome bands in chromosome arm 3L of Drosophila melanogaster includes the loci of two abundant developmentally regulated larval proteins. The structural gene for larval serum protein 2 (LSP 2) lies at 68E3 or 4, and that for salivary glue secretion protein 3 between 68A8 and 68C11, coincident with a major intermoult puff active in the salivary gland at the time of glue synthesis. The structural genes for esterase 6 and four visible recessive loci lie within the same region.

The developmental profiles of two major haemolymph proteins from Drosophila melanogaster.

Abstract There are four major protein species in the haemolymph of the late 3rd instar of Drosophila . Two of these, LSP-1 and LSP-2, have been studied in detail. Larvae, pupae, and flies of different ages were measured for wet weight, total extractable protein and using an immunoassay, the amounts of LSP-1 and LSP-2. The synthesis of both proteins begins after the 2nd larval ecdysis and at puparium formation they represent 9% and 1.5% of total extractable protein. This value remains constant during the first part of metamorphosis, then falls rapidly. The function of these proteins and their suitability as systems for the study of gene control and protein synthesis in Drosophila are discussed.

The segmentation cascade in the centipede Strigamia maritima: Involvement of the Notch pathway and pair-rule gene homologues

The centipede Strigamia maritima forms all of its segments during embryogenesis. Trunk segments form sequentially from an apparently undifferentiated disk of cells at the posterior of the germ band. We have previously described periodic patterns of gene expression in this posterior disc that precede overt differentiation of segments, and suggested that a segmentation oscillator may be operating in the posterior disc. We now show that genes of the Notch signalling pathway, including the ligand Delta, and homologues of the Drosophila pair-rule genes even-skipped and hairy, show periodic expression in the posterior disc, consistent with their involvement in, or regulation by, such an oscillator. These genes are expressed in a pattern of apparently expanding concentric rings around the proctodeum, which become stripes at the base of the germ band where segments are emerging. In this transition zone, these primary stripes define a double segment periodicity: segmental stripes of engrailed expression, which mark the posterior of each segment, arise at two different phases of the primary pattern. Delta and even-skipped are also activated in secondary stripes that intercalate between primary stripes in this region, further defining the single segment repeat. These data, together with observations that Notch mediated signalling is required for segment pattern formation in other arthropods, suggest that the ancestral arthropod segmentation cascade may have involved a segmentation oscillator that utilised Notch signalling.

HOM/Hox genes of Artemia: implications for the origin of insect and crustacean body plans

BACKGROUND Insects and crustaceans are generally assumed to derive from a segmented common ancestor that had a distinct head but uniform, undifferentiated trunk segments. The subdivision of the body into functionally distinct regions (e.g. thorax and abdomen) is thought to have evolved independently in these two lineages. In insects, the differences between segments in the trunk are controlled by the Antennapedia-like genes of the homeotic gene clusters. Study of these genes in crustaceans should provide a basis for comparing body plans and assessing their evolutionary origin. RESULTS Using a polymerase chain reaction (PCR) / inverse PCR strategy, we have isolated six genes of the HOM/Hox family from the crustacean Artemia franciscana. Five of these are clearly identifiable as specific homologues of the insect homeotic genes Dfd, Scr, Antp, Ubx and abdA. The sixth appears to have no close counterpart in insects. CONCLUSION All the homeotic genes that specify middle body regions in insects originated before the divergence of the insect and crustacean lineages, probably not later than the Cambrian (about 500 million years ago). A commonly derived groundplan may underlie segment diversity in these two groups.

Hox gene expression in larval development of the polychaetes Nereis virens and Platynereis dumerilii (Annelida, Lophotrochozoa).

The bilaterian animals are divided into three great branches: the Deuterostomia, Ecdysozoa, and Lophotrochozoa. The evolution of developmental mechanisms is less studied in the Lophotrochozoa than in the other two clades. We have studied the expression of Hox genes during larval development of two lophotrochozoans, the polychaete annelids Nereis virens and Platynereis dumerilii. As reported previously, the Hox cluster of N. virens consists of at least 11 genes (de Rosa R, Grenier JK, Andreeva T, Cook CE, Adoutte A, Akam M, Carroll SB, Balavoine G, Nature, 399:772–776, 1999; Andreeva TF, Cook C, Korchagina NM, Akam M, Dondua AK, Ontogenez 32:225–233, 2001); we have also cloned nine Hox genes of P. dumerilii. Hox genes are mainly expressed in the descendants of the 2d blastomere, which form the integument of segments, ventral neural ganglia, pre-pygidial growth zone, and the pygidial lobe. Patterns of expression are similar for orthologous genes of both nereids. In Nereis, Hox2, and Hox3 are activated before the blastopore closure, while Hox1 and Hox4 are activated just after this. Hox5 and Post2 are first active during the metatrochophore stage, and Hox7, Lox4, and Lox2 at the late nectochaete stage only. During larval stages, Hox genes are expressed in staggered domains in the developing segments and pygidial lobe. The pattern of expression of Hox cluster genes suggests their involvement in the vectorial regionalization of the larval body along the antero-posterior axis. Hox gene expression in nereids conforms to the canonical patterns postulated for the two other evolutionary branches of the Bilateria, the Ecdysozoa and the Deuterostomia, thus supporting the evolutionary conservatism of the function of Hox genes in development.