Echinoderm conundrums: Hox genes, heterochrony, and an excess of mouths.

Thurston Lacalli

doi:10.1186/2041-9139-5-46

Abstract

Two issues relating to the translocation of anterior Hox genes in echinoderms to the 5’ end of the Hox cluster are discussed: i) that developmental changes associated with fixation to the substratum have led to an acceleration of mesodermal development relative to that of ectoderm, resulting in a mismatch of anteroposterior registry between the two tissues and a larger role for mesoderm in patterning control, and ii) whether this helps explain the ability of some echinoderms to form separate mouths at different locations, one for the larva and one for the juvenile rudiment. Freeing the mesoderm from ectodermal influences may have encouraged morphogenetic innovation, paralleling the situation in tunicates, where an early genomic (or genomic and developmental) change has allowed the body to evolve in novel ways.

Highlights

This essay began as a brief Comment on an informative review of echinoderm Hox genes by David & Mooi recently published in this journal [1], but morphed into a somewhat broader treatment of echinoderm development
David & Mooi argue this divergence correlates with translocation of the anterior Hox genes to the 5’ end of the cluster, along with their inversion, so as to take them completely out of the game when it comes to early patterning events
The case made by David & Mooi suggests an alternative that reverses the polarity of this scenario, so that asexual propagation would be a consequence of a deeper developmental genetic change that allowed for subsequent alteration of the way mouth position is determined

Summary

Background

Heterochrony and its consequences My first elephant is the restriction of early Hox expression to mesodermal structures, namely coeloms, in patterns that conserve the ancestral linear anteroposterior (A/P) sequence within the mesoderm. A rationale for seeing this as related to alterations in body plan is as follows: in all deuterostomes except echinoderms, the ectoderm and mesoderm show a high degree of A/P registry, that is, defining “anterior” from the developmental genetic perspective as being the site of the larval apical organ, a key landmark across phyla [5], the A/P identity of adjacent tissue layers is roughly in register along the whole length of the body, and development follows an anterior to posterior sequence. The case made by David & Mooi suggests an alternative that reverses the polarity of this scenario, so that asexual propagation (which occurs in some echinoderm larvae, see [18]) would be a consequence of a deeper developmental genetic change that allowed for subsequent alteration of the way mouth position is determined This is comparable in some ways to the fundamental developmental and genomic change postulated to have freed tunicates to evolve a whole set of body plan innovations [19]. From the perspective of an amphioxus biologist, where both the genome and morphology are comparatively conservative, the ability of echinoderms to remake their morphology with such apparent ease is remarkable

Conclusions

Holland ND: Echinodermata

14. Lacalli TC

19. Holland LZ