Analyses of segment-specific expression of alkaline phosphatase activity in the mesoderm of the oligochaete annelid Tubifex: Implications for specification of segmental identity

In the embryos of the oligochaete annelid Tubifex, segments VII and VIII sp ecifically express mesodermal alkaline phosphatase (ALP) activity in the ve ntrolateral region. In this study, we examined whether this segment-specifi c expression of ALP activity depends on external cues. Cell lineage analyse s show that the ALP-expressing cells originate from M teloblasts. Furthermo re, a set of teloblast-ablation experiments demonstrated that the seventh a nd eighth primary m blast cells (m7 and m8) produced from M teloblasts give rise to ALP-expressing cells in segments VII and VIII, respectively, and t hat primary m blast cells other than m7 and m8 lack the ability to generate ALP-expressing progeny cells. The results of another set of blastomere-abl ation experiments suggest that ALP-expressing cells emerge independently of interactions with surrounding tissues. Teloblast-transplantation experimen ts demonstrated that mg can generate ALP-expressing cells in an ectopical p osition, suggesting that it is unlikely that ALP activity emerges in respon se to the positional cues residing in the embryo. These results suggest tha t m7 and m8 are exclusively specified as precursors of ALP-expressing cells at the time of their birth from RI teloblasts. We propose that segmental i dentities in primary m blast cells of the Tubifex embryo are determined acc ording to the genealogical position in the M lineage and that the M telobla st possesses a developmental program through which the sequence of blast ce ll identities is determined. (C) 2000 Academic Press.