Hislop, Nikki Rochelle (2003) Characterisation of anterior patterning genes in the staghorn coral, Acropora millepora(Cnidaria; Anthozoa; Scleractinia). PhD thesis, James Cook University.

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Abstract

To a surprising extent, molecular mechanisms underlying many aspects of development are conserved across the higher (bilateral and triploblastic) Metazoa, but the degree to which similar mechanisms apply to non-bilateral and diploblastic animals such as cnidarians is unclear. The aim of this thesis was to characterise genes homologous to those that play key roles in anterior patterning in bilaterians, in the basal cnidarian Acropora millepora. In Acropora, a gene clearly related to the ems/Emx family was shown to be expressed in a subset of presumed neurons, restricted along the oral-aboral (O/A) axis to the aboral end of the planula larva, in a pattern overlapping that of cnox2-Am, the Acropora ind/Gsx homolog. Hence, key components of both anterior-posterior (A/P) and dorsalventral (D/V) patterning systems of higher animals are differentially expressed along the single (O/A) overt body axis of the Acropora planula. During metamorphosis, emx- Am expression is significantly reduced, corresponding to the degeneration of the nervous system observed prior to settlement (Ball et al., unpublished). After settlement, expression of emx-Am is again detected in a subset of neurons, although an axis-related pattern of distribution is not apparent. Yeast-one hybrid and electrophoretic mobility shift assays (EMSA) were used to identify candidate target sites for the Emx-Am homeodomain and to examine its DNA-binding behaviour in vitro. Yeast-one hybrid experiments identified three regions within the Acropora genome containing putative Emx-Am binding sites, and further characterisation of these regions attempted to identify potential downstream targets of the Emx-Am homeodomain. Data regarding downstream targets of Ems/Emx proteins is limited, but EMSA provided novel evidence that Ems/Emx homeodomains may be able dimerise on TAAT half sites, a characteristic feature of many ANTP-related proteins. In Drosophila, the Tailless nuclear receptor is a direct regulator of ems; for this reason the expression of tlx-Am, an Acropora gene encoding a protein clearly related to Tailless, was studied. In addition to its patterns of expression, the tlx-Am locus was also characterised, but this analysis revealed that a similar regulatory relationship with ems is unlikely to exist in the coral. Attempts to clone the Acropora homolog of a Hydra prdl-a gene led to the fortuitous identification of four paired-like genes. Although none of these appears to be strictly orthologous with prdl-a, the sequences and expression patterns of these genes provided some novel insights into the evolution of the Paired superfamily of homeodomain proteins. One of these genes (hbn-Am) is orthologous with Drosophila homeobrain, and appears to be a very early ectodermal marker during coral development. hbn-Am is expressed in a region corresponding to the presumptive ectoderm, in a pattern that is complimentary and mutually exclusive to that of the coral snail homolog (snail-Am), suggesting that these genes interact. Two of the paired-like genes identified are clearly related and were shown to be organised in tandem in the Acropora genome. Two other cases of tightly linked pairs of related genes were also identified, suggesting that a high proportion of coral genes may be organised in this way.

Item ID:	80
Item Type:	Thesis (PhD)
Keywords:	Acropora Millepora, Cnidarians, Anterior patterning genes, Staghorn coral, Cnidaria, Anthozoa, Scleractinia
Date Deposited:	03 Aug 2006
FoR Codes:	06 BIOLOGICAL SCIENCES > 0606 Physiology > 060603 Animal Physiology Systems @ 50% 06 BIOLOGICAL SCIENCES > 0604 Genetics > 060403 Developmental Genetics (incl Sex Determination) @ 50%
SEO Codes:	96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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