Embryonic expression of HeFoxA1 and HeFoxA2 in an indirectly developing polychaete.

Two forkhead family transcription factors, HeFoxA1 and HeFoxA2, were isolated from the serpulid annelid Hydroides elegans and their transcript distribution were characterized during embryogenesis. HeFoxA1 is first detected in second quartet blastomeres soon after their formation, and later in all vegetal half blastomeres, which comprise ectoderm, endoderm, and mesoderm precursors. HeFoxA1 expression declines first in subtrochal ectoderm and presumptive midgut precursors, as well as apparently in D quadrant blastomeres in advance of any known signaling events. Later, during gastrulation, HeFoxA1 declines in hindgut precursors, and by the end of gastrulation the expression remains active only in foregut precursors. HeFoxA1 is apparently expressed in ectomesoderm cells involved in forming the larva-specific protonephridium (the so-called head kidney). The other ortholog, HeFoxA2, is expressed in a subset of the cells in which HeFoxA1 is expressed during early stages, but later it is largely restricted to the endoderm-ectoderm boundary of the proctodaeum. In addition, HeFoxA2 has a unique expression in two hindgut cells and abutting ectoderm cells located by the imminent anal opening. The combined expression of HeFoxA1 and HeFoxA2 correlates with mesoderm and endoderm expression of their orthologs in other bilaterians.