Evolutionary developmental biology is revealing an underlying conservation of numerous developmental regulatory genes in evolutionary disparate taxa that exhibit overt differences in morphology. This is exemplified by the nervous system, which is a panbilaterian structure that expresses orthologous genes in a variety of taxa.
The tropical abalone, Haliotis aisnina, is presented here as a representative lophotrochozoan in which the expression of genes conserved through evolution are investigated. Using the candidate gene approach orthologues of POU (HasPOU-III, HasPOU-IV), Pax (HasPax-258, HasPax-6) and Sox (HasSox-B, HasSox-C) transcription factor genes were isolated from the central nervous system of H. asinina. The sequences align closely with other bilaterian representatives and phylogenetic analyses of these genes indicate that they predate the bilaterian divergence.
Analysis of temporal expression, reveals that HasSox-B, HasSox-C and HasPax-258 are initially maternally expressed suggesting a role in embryogenesis. As with several other H. asinina developmental genes, transcription of HasPOU-III, HasPOU-IV and HasPax-6 is initiated by the trochophore stage when organogensis of adult structures is initiated.
To facilitate interpretation of gene expression during larval development, the serotonergic immunoreactive cells of the larvae were investigated. All larval and adult ganglia express serotonin during larval development and are consistent with expression in other molluscs. The ganglia of the developing adult central nervous system, including the cerebral, pleuropedal, branchial and oesophageal ganglia, also express Class III and class IV POU genes in H. asinina
A range of cells and tissues in the developing peripheral nervous system also express HasPOU-III, HasPOU-IV and HasPax-258 including the chemo- and mechanosensory cephalic and epipodial tentacles and the branchial chamber. In particular, the conservation of Pax-258, POU-III and POU-IV orthologues in the statocyst and in mechanosensory structures highlights the antiquity of a mechanosensory geotactic-like organ in the Bilateria. The association of these transcription factor genes with the nervous system is retained in the adult. HasPOU-III, HasPOU-IV, HasPax-258, HasPax-6, HasSox-B, HasSox-C are expressed in a wide range of tissues in the abalone central and peripheral nervous system including the anterior ganglia, eyes, tentacles, gill and epipodial fringe. However, RT-PCR analysis of HasPOU-III, HasPOU-IV and HasSox-C expression indicates downregulation in the presence of a digenean parasite shown to ‘castrate’ adult abalone .
Besides expression in the nervous system, HasPOU-III is expressed in the mucous cells of the foot, caudal cells of the visceral mass, in the secretory region of the statocysts and the presumptive developing secretory sheath of the radula of the developing H. asinina and in the adult, the renal gland, digestive gland and mantle. Class III POU genes are expressed in equivalent developing and functional bilaterian secretory organs indicating conservation throughout evolution.
Comparison of POU and Pax gene expression between taxa from the three bilaterian superphyla reveals spatial conservation, particularly in the developing and adult nervous system.