Evolution of the micromeres in echinoids Takuya Minokawa Research Center for Marine Biology, Tohoku University The formation of “micromere quartet” in the vegetal-most region of the 16-cell-stage embryo is one of the common embryonic features of Euechinoidea (a group of modern echinoids). Two different functions of the micromeres and its descendants have been reported. First, they differentiate autonomously into skeletogenic mesenchyme cells. Second, the micromeres and its descendants send signals to induce endomesoderm in neighboring cells. These functions have established and modified during the divergence of echinoids. We study various echinoid model systems (the “primitive” Cidaroidea echinoid Prionocidaris baculosa, the “derived” direct developing echinoid Peronella japonica, and others) to gain an understanding of the molecular and cellular mechanisms that underlie the evolution of micromere specification and functions. The comparative study between Cidaroidea and Euechinoidea allows us to estimate the ancestral mode of the larval skeletogenic mechanism. The 16-cell-stage Prionocidaris embryos do not form “micromere quartet”. The expression patterns of several skeletogenesis-related genes in Prionocidaris are different from that of the Euechinoidea orthologs and resemble to that of the starfish orthologs, suggesting that Prionocidaris preserves the ancestral mechanism for larval skeletogenesis of the echinoid. The comparison between the indirect- and the direct-developing echinoids allows us to study the mechanical basis of the evolution of a novel developmental program. The micromeres in Peronella embryo do not express the inductive signals for endomesoderm. Several endomesoderm specification genes in Peronella show modified expression patterns. Based on these comparative studies, the evolution of micromeres and its function will be discussed.

Evolution of the micromeres in echinoids

Takuya Minokawa

Research Center for Marine Biology, Tohoku University

The formation of “micromere quartet” in the vegetal-most region of the 16-cell-stage embryo is one of the common embryonic features of Euechinoidea (a group of modern echinoids). Two different functions of the micromeres and its descendants have been reported. First, they differentiate autonomously into skeletogenic mesenchyme cells. Second, the micromeres and its descendants send signals to induce endomesoderm in neighboring cells. These functions have established and modified during the divergence of echinoids. We study various echinoid model systems (the “primitive” Cidaroidea echinoid Prionocidaris baculosa, the “derived” direct developing echinoid Peronella japonica, and others) to gain an understanding of the molecular and cellular mechanisms that underlie the evolution of micromere specification and functions.

The comparative study between Cidaroidea and Euechinoidea allows us to estimate the ancestral mode of the larval skeletogenic mechanism. The 16-cell-stage Prionocidaris embryos do not form “micromere quartet”. The expression patterns of several skeletogenesis-related genes in Prionocidaris are different from that of the Euechinoidea orthologs and resemble to that of the starfish orthologs, suggesting that Prionocidaris preserves the ancestral mechanism for larval skeletogenesis of the echinoid.

The comparison between the indirect- and the direct-developing echinoids allows us to study the mechanical basis of the evolution of a novel developmental program. The micromeres in Peronella embryo do not express the inductive signals for endomesoderm. Several endomesoderm specification genes in Peronella show modified expression patterns.

Based on these comparative studies, the evolution of micromeres and its function will be discussed.