. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. CHAETOPTERUS LARVAL ONTOGENY 329 niban et <//., 1997). Our observations never give the impres- sion that these segmental rudiments are produced sequen- tially, anterior to posterior, from a mesodermal band or bands. We propose the hypothesis, supported by all the existing data from this and other studies, that the anterior 15 seg- ments in Cluietopterus are formed by subdivision of exist- ing anlage, rather than by sequential addition from a growth zone. The segments B1-B5 are formed first, at stage L4, by subdivision of
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. CHAETOPTERUS LARVAL ONTOGENY 329 niban et <//., 1997). Our observations never give the impres- sion that these segmental rudiments are produced sequen- tially, anterior to posterior, from a mesodermal band or bands. We propose the hypothesis, supported by all the existing data from this and other studies, that the anterior 15 seg- ments in Cluietopterus are formed by subdivision of exist- ing anlage, rather than by sequential addition from a growth zone. The segments B1-B5 are formed first, at stage L4, by subdivision of the stage L3 coeloms II and III. Segments AI-A9 form later, at stage L5, by subdivision of the stage L3 coelom I. This hypothesis does not rule out the produc- tion of the segmented body elements from a teloblastic growth zone, but does temporally dissociate the production of those elements from their morphogenesis as discrete segmental structures. Other recent work has shown that Hox gene expression begins as early as stage L2 in the putative growth zone (Irvine, 1998; Irvine and Martindale. 1999b; Kevin J. Peterson, pers. comm.). If the Hox genes are acting as segmental specification genes at this stage, this early onset of expression suggests that the delay in the appearance of overt segmentation in Chaetopterus is the result of a lengthening of the period between molecular specification of segments and their morphogenesis. Testing the accuracy of this model will require more extensive analysis of inter- vening stages, possibly using histological sections or cell- labeling techniques. Even if the correlation of early larval structures with their final segmental products differs from what we have pro- posed, the timing and nature of segmental differentiation in Chaetopterus remain highly diverged relative to the patterns seen in related polychaete families. This is not surprising given the extreme level of adult body plan divergence. What is remarkable is how far back into la
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
