Cell-specific gene regulation in early molluscan development

Abstract

In order to understand the processes that regulate cell-specific gene expression in early molluscan development, we studied the expression of tubulin genes in embryos of Patella vulgata (Gastropoda, Molluscs). Tubulin genes are fust expressed at the 32-cell stage, which is also the stage at which the embryonic transcription starts. At this stage expression of tubulin genes is observed exclusively in two cells in each quadrant, the trochoblasts. These cells, after one additional division, will differentiate into ciliated and cleavage arrested cells. Later they will form part of the prototroch, the locomotory organ of the fiee-swimming trochophore larva. A trochoblast-specific a-tubulin gene was cloned and its 5' upstream region was fused to the lac-Z reporter gene. After micro-injection into 2-cell stage embryos, the expression of this fusion gene appears to be restricted to the trochoblasts at the right time. Mutagenic analysis showed that two elements located between -108 and -42 were essential for a correct spatiotemporal expression. The fust element, which contains a putative recognition site for homeobox proteins, acts negatively, the second one positively. In south-westem and gel retardation assays we could show that several nuclear proteins are able to bind to these elements. The molecular mechanisms that regulate trochoblast-specific gene expression appear to be evolutionarily highly conserved. The 5' region of the Patella tubulin gene is not only cell type specifically activated in embryos of this gastropod, but also in embryos of spirally cleaving species from other taxonomic groups, e.g., in a polyplacophoran, a scaphopod, and an annelid.

Bilaterian Origins: Significance of New Experimental Observations

“…The conclusion that similarities among trochophore larvae are the result of convergent evolution can be challenged experimentally, and it is not necessary to address the paleontological interpretations on which this scenario is based. Thus, for example, and Damen et al (1997) showed convincingly that the trochophores of annelids and molluscs share detailed similarities in the genetic regulatory circuitry underlying the development of trochophore-specific features. These features can hardly have arisen by convergence.…”

“…Peterson et al, unpublished data). Most importantly, Damen et al (1997;see also van den Biggelaar et al, 1997) showed that the 5Ј upstream region of the Patella ␤-tubulin gene will correctly drive expression of a reporter construct not only in the trochoblasts of other molluscan embryos (including a polyplacophoran and a scaphopod), but in the trochoblasts of polychaete annelid embryos as well. Hence, the trochoblasts of annelid and molluscan embryos express the same lineage-specific gene, and expression is driven by the same set of transcription factors.…”

Indirect development, transdifferentiation and the macroregulatory evolution of metazoans

“…The prototroch is the primary ciliary band of spiralians (Nielsen 2005b). In support of ancestral regulatory entities, cis-regulatory sequences of a gastropod alpha-tubulin gene drive trochoblast expression in the primary ciliary band of various spiralians (Damen et al 1997). Secondary ciliary bands have different lineage origins among spiralians (Arenas-Mena et al 2007b;Henry et al 2007), although different lineages do not preclude deeper regulatory homology.…”

Origin of the trochophora larva

“…The few studies on gene expression, e.g. on Patella [ 23 ] and Hydroides [ 20 , 24 , 25 ], are too scattered to contribute to the discussion.…”

“…The few studies on gene expression, e.g. on Patella [23] and Hydroides [20,24,25] cells. The four apical cells are called the first micromere quartet (called 1a-1d or collectively 1q, their descendants are called 1q 1 and 1q 2 ) is referred to as the episphere (figure 2c).…”