Cell Fate Determination Along The Anterior-Posterior Axis In The Mud Snail Ilyanassa obsoleta.
Previous studies in the mud snail, Ilyanassa obsoleta, described a novel centrosome- dependent mechanism to differentially distribute mRNAs within the embryo during the first 7 cleavage cycles of development (Lambert and Nagy 2002). I hypothesized that the simplest explanation to account for the observed distribution patterns would be to regulate the timing of the transcription of individual mRNAs in the stem cells. I found that unique patterns of centrosomal localization do not depend on transcription or RNA concentration. All measured centrosomally localized mRNAs are maternally provided at a relatively uniform and high concentration and are long lived. I show that centrosomal localization does not protect mRNA from degradation. I also discovered that the onset of transcription in Ilyanassa roughly coincides with the degradation of maternal transcripts in a maternal-zygotic transition (MZT) event that does not occur until long after gastrulation. In addition, because centrosomally localized mRNAs are inherited to specific quartets of cells that are always born towards the animal pole, and more animal quartets make more anterior fates, I hypothesized that this mechanism would be used for patterning cell fates along the anterior-posterior (A/P) axis. I chose to look at the expression of the Hox genes, a highly conserved family of genes involved in axial patterning within bilateral organisms. I cloned 11 Hox genes fragments from the gastropod Ilyanassa obsoleta and compared the Ilyanassa sequences to the released genomes of Capitella capitata and Lottia gigantean. I find that both of these species have a single continuous Hox cluster with 10 and 11 genes, respectively. Gene order of the anterior class genes within these two Hox clusters is similar to that predicted from insects and vertebrates, while the order of two central class genes and the posterior class genes are inverted. I find no evidence in Ilyanassa for the striking spatial co-linearity of Hox gene expression common in other animals. Lastly, I have found the posterior group gene Iob Post2 is expressed exclusively in cells known to produce the shell. My data suggests that Iob Post2 has been recruited during molluscan evolution to produce a morphological feature that is a unique feature of the molluscs.
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