Meeting Abstract
Leeches are active epibenthic predators or ectoparasites; in contrast, their ancestors and close extant relatives (oligochaetes) are infaunal detritus feeders. In the transition from oligochaetes, leeches evolved novel features, e.g., determinate growth (32 segments, all arising in embryogenesis), anterior and posterior suckers, and a specialized midgut with segmental ceca. Leeches and oligochaetes also exhibit massive genomic changes (gene loss & duplication, loss of macrosynteny) relative to other animals. For example, the leech Hox cluster is highly atomized, and has multiple gene losses and duplications relative to the ancestral lophotrochozoan. Has Hox cluster disorganization enabled new regulatory interactions and functions for these transcription factors? Do these molecular novelties contribute to the morphological novelties in leeches? We use the tractable leech species Helobdella austinensis as a model in which to characterize the expression, function and regulation of duplicated and single-copy Hox genes. In contrast to arthropods and vertebrates, CRISPR-induced Hox mutants in leech do not show overt homeosis (altered segment identity). Rather, we observe: defects in gut and sucker formation (for post2a, post2c, lox4b); late-embryonic edema (for lox5), possibly due to a failure in kidney development; an embryonic-lethal phenotype during epiboly (for hox3), perhaps caused by duplication of segmentation stem cells (teloblasts); and new gene expression patterns and regulatory interactions compared with those known in polychaetes. Our results reveal how Hox-related developmental mechanisms may have contributed to annelid body plan diversification.
