HeOtx expression in an indirectly developing polychaete correlates with gastrulation by invagination

Abstract

The expression of an Otx homolog in the indirectly developing polychaete Hydroides elegans was characterized during embryo, trochophore, and feeding-larva stages. In the animal hemisphere, HeOtx is first expressed in 1q(12) blastomeres and their immediate descendants. Such discrete embryonic animal hemisphere Otx expression perhaps relates to cell-type specification functions of the larva. During feeding stages, transcripts are detected in adult cerebral ganglia precursors and putative adult eye precursors, where it may have adult brain regionalization functions. HeOtx is not expressed in primary trochoblast precursors, but it is expressed in cells adjacent to the ciliary band. HeOtx is also expressed in a group of cells in the dorsal midline of the early trochophore larva in putative posterior sensory organ precursors. The vegetal hemisphere expression starts in oral and lateral sides of the blastopore and later expands to central blastomeres that lead the gastrulation movements. During late gastrulation stages, the expression declines in foregut precursors, but it is maintained in midgut precursors, suggesting its involvement in tripartite gut subdivision functions. HeOtx broader and earlier endoderm expression correlates with gastrulation by invagination associated with the formation of the feeding trochophore, in contrast with a later and orally restricted Otx expression found in a polychaete that gastrulates by epiboly and forms a non-feeding trochophore. The endoderm expression and functional roles in other bilaterians suggest an ancestral role of Otx related to gastrulation by invagination.

Development of a feeding trochophore in the polychaete Hydroides elegans

“…Remarkably, all the transcription factors characterized so far initiate their expression in strikingly precise association with the invariant cleavage (Arenas-Mena, 2006Arenas-Mena and Wong 2007). For example, FoxA1 initiates its expression 64-cell embryo, nuclear staining (Arenas-Mena, 2007b (Bastida-Zavala and Ten Hove, 2003).…”

“…For some genes, invariable gene expression correlates with immediate differentiation, such as the animal cap blastomeres expressing Otx, which apparently differentiate into neurons ( Fig. 3) (Arenas-Mena and Wong, 2007), or the restriction of T-brain to apical tuft sensory precursors ( Fig. 3) (Arenas-Mena, 2008).…”

“…2). The animal-vegetal compartments are partially blurred by animal cap cells that apparently migrate from the animal to the vegetal hemispheres through a small dorsal gap in the ciliary band (Mead, 1897;Treadwell, 1901;Arenas-Mena and Wong, 2007). Subsequent to spiral cleavage, gastrulation and ectoderm (Anderson, 1966) (Anderson, 1966;Arenas-Mena, 2007b).…”

“…2) (Anderson, 1966). Transcription factors FoxA, Brachyury, T-brain, Blimp, Otx and Sall are expressed in the vegetal hemisphere in dynamic patterns that correlate with morphogenetic events associated with gastrulation by invagination (Arenas-Mena 2006Arenas-Mena and Wong, 2007). FoxA and Brachyury form a synexpression group associated with gastrulation by invaginaby epibolic gastrulation does not require their expression.…”

“…Similarly, neighboring sisters 1q22 and 1q21 elegantly divide in complementary asymmetric patterns in each quadrant to invariably position in the same plane the eight primary trochoblasts 1q221 and 1q212 that are much larger than their respective vegetal 1q222 and animal 1q211 sisters . It remains uncertain if cells undergoing symmetric cell divisions similarly correspond to invariant fates, or if more flexibility and/or later specification ensues in these lineages.Remarkably, all the transcription factors characterized so far initiate their expression in strikingly precise association with the invariant cleavage (Arenas-Mena, 2006Arenas-Mena and Wong 2007). For example, FoxA1 initiates its expression 64-cell embryo, nuclear staining (Arenas-Mena, 2007b (Bastida-Zavala and Ten Hove, 2003).…”