Specification of embryonic axis and mosaic development in ascidians

Abstract

Setting up future body axes is the first important event before and at the beginning of embryogenesis. The ascidian embryo is a classic model that has been used to gain insight into developmental processes for over a century. This review summarizes advances made in this decade in our understanding of the developmental processes involved in the specification of the embryonic axes and cell fates during early ascidian embryogenesis. Maternal factors, including mRNAs, are translocated to specific regions of the egg by cytoplasmic and cortical reorganization, so-called ooplasmic segregation, and specify the animal-vegetal axis and the one perpendicular to it, which is defined as the anteroposterior axis in ascidians. Some postplasmic/PEM RNAs that are anchored to cortical endoplasmic reticulum are brought to the future posterior pole of fertilized eggs, and play crucial roles in posterior development. Following specification of the animal-vegetal axis, nuclear localization of ␤-catenin takes place in the vegetal blastomeres; this occurrence is important for the acquisition of the vegetal character of the blastomeres in later development. Positioning of these maternal factors lead to subsequent cell interactions and zygotic gene expression responsible for axis establishment and for cell fate specification. We describe how endoderm blastomeres in the vegetal pole region emanate inductive signals mainly attributable to fibroblast growth factor. Marginal blastomeres next to endoderm blastomeres respond differently in ways that are determined by intrinsic competence factors. Expression patterns of developmentally important genes, including key transcription factors of each tissue type, are also summarized.

Embryology of a planktonic tunicate reveals traces of sessility

“…S3). The second cleavage is traditionally thought to establish the anterior-posterior axis in tunicates (18)(19)(20)(21). Our results, however, demonstrate that what traditionally was labeled ''anterior'' coincides with the prospective nervous system in early stages in O. dioica (Figs.…”

An Advanced Filter-Feeder Hypothesis for Urochordate Evolution

“…The CAB is present in ascidians (Hibino et al, 1998;Nishikata et al, 1999), and a similar structure was also Figure S1 online for a similar diagram with blastomere names and cell fates superimposed. For details of the spatial arrangements of blastomeres, see Satoh (1979), Nishida (2005Nishida ( , 2008, and Fujii et al (2008). Ani, animal pole.…”

“…Inferences about the basic features of embryogenesis in chordates can be deduced from these fate map similarities. Chordates share similar fate maps, show similar morphogenetic movements during gastrulation and neurulation (Nishida, 2005), and eventually develop into tadpole-shaped larvae with similarly organized tissues and organs. They commonly have a dorsal neural tube and central notochord flanked by bilateral muscle in the tail.…”

A Developmental Basis for the Cambrian Radiation

“…Cellular-level analysis of regional specification in urochordates and cephalochordates that are descendants of stocks that originated during the Lower Cambrian (Chen et al, 2003 for ascidians;Mallatt and Chen, 2003 for cephalochordates) indicates that many features of the pattern of regional specification in these two subphyla are quite different (Satoh, 1994 andNishida, 2005 for ascidians; Whittaker, 1997 for lancets; Swalla, 2004 for both groups). Recently a gene regulatory network for early development in ascidians was published for Ciona (Iami et al, 2006).…”