ISOLATION, CHARACTERIZATION, AND EXPRESSION OF LE-MSX, A MATERNALLY EXPRESSED MEMBER OF THE MSX GENE FAMILY FROM THE GLOSSIPHONIID LEECH, HELOBDELLA

The msx gene family is one of the most highly conserved of the nonclus tered homeobox-containing genes. We have isolated an msx homolog (Le-m sx) from the glossiphoniid leech, Helobdella robusta, and characterize d its pattern of expression by whole mount in situ hybridization. In s itu expression and reverse transcription polymerase chain reaction (RT -PCR) data results show that Le-msx is a maternal transcript initially uniformly distributed in the cortex of immature oocytes that becomes asymmetrically localized to the polar regions of the uncleaved zygote. This is the earliest reported expression for the msx gene family and the first maternally expressed homeodomain-containing transcription fa ctor reported in annelids. During embryonic development, Le-msx is exp ressed in all 10 embryonic stem cells and their segmental founder cell descendants. At midembryonic stages, Le-msx is expressed in the expan ding germinal plate. Le-msx is confined to the central nervous system and nephridia at late (stage 9) stages and subsequently disappears fro m nephridia. In addition, we present a phylogenetic hypothesis for the evolution of the msx gene family, including the identification of a p utative C. elegans msx homolog and the realignment of the sponge msx h omolog to the NK class of homeodomain genes. (C) 1996 Wiley-Liss, Inc.