Progressive activation of Delta-Notch signaling from around the blastopore is required to set up a functional caudal lobe in the spiderAchaearanea tepidariorum

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Abstract
In the development of most arthropods, the caudal region of the elongating germ band (the growth zone) sequentially produces new segments. Previous work with the spider Cupiennius salei suggested involvement of Delta-Notch signaling in segmentation. Here, we report that, in the spider Achaearanea tepidariorum, the same signaling pathway exerts a different function in the presumptive caudal region before initiation of segmentation. In the developing spider embryo, the growth zone becomes morphologically apparent as a caudal lobe around the closed blastopore. We found that, preceding caudal lobe formation, transcripts of a Delta homolog, At-Delta, are expressed in evenly spaced cells in a small area covering the closing blastopore and then in a progressively wider area of the germ disc epithelium. Cells with high At-Delta expression are likely to be prospective mesoderm cells, which later express a twist homolog, At-twist, and individually internalize. Cells remaining at the surface begin to express a caudal homolog, At-caudal, to differentiate as caudal ectoderm. Knockdown of At-Delta by parental RNA interference results in overproduction of At-twist-expressing mesoderm cells at the expense of At-caudal-expressing ectoderm cells. This condition gives rise to a disorganized caudal region that fails to pattern the opisthosoma. In addition, knockdown of Notch and Suppressor of Hairless homologs produces similar phenotypes. We suggest that, in the spider, progressive activation of Delta-Notch signaling from around the blastopore leads to stochastic cell fate decisions between mesoderm and caudal ectoderm through a process of lateral inhibition to set up a functional caudal lobe.