Meeting Abstract

P3.35  Jan. 6  Uncoupling of dorsal and ventral segmentation systems in the branchiopod crustacean Triops longicaudatus VANHOOK, A.M.*; PATEL, N.H.; Univ. of California, Berkeley, and Howard Hughes Medical Institute; Univ. of California, Berkeley, and Howard Hughes Medical Institute annalisa@berkeley.edu

An extreme departure from the orthodox arthropod segmentation pattern is observed in the tadpole shrimp, Triops longicaudatus. In this crustacean, there is a discontinuity of ventral and dorsal segments. In the anterior trunk, dorsal and ventral portions of segments are aligned and continuous along the DV axis. Posterior to the genital (11th postgnathal) segment, ventral segments begin a gradual decrease in size while dorsal segments remain constant in size. Consequently, dorsal and ventral segments become unaligned, and the animal appears to have two entirely separate segmental arrays. At the posterior end of the animal, there may be as many as 7 ventral segments per dorsal one, and there is no quantal relationship between the number of dorsal and ventral segments posterior to the genitals. We have shown that both dorsal and ventral morphological units in Triops are true segments as defined by segment polarity gene expression (i.e. fusion events are not the cause of Triops� unusual segmentation). The segmenting regions are uncoupled, but segmentation proceeds by the same basic mechanism in both dorsal and ventral regions. The difference in segment size arises from the difference in cell size when the segments are formed. Ventral segments are formed from cells that gradually but dramatically decrease in size posteriorly, while dorsal segments are formed from a pool of cells of constant size. We hypothesize that different cell cycle rates in dorsal and ventral segmenting tissue are responsible for this difference in segment size, and experiments with cell cycle inhibitors support this hypothesis.