The single MyoD family gene of Ciona intestinalis encodes two differentially expressed proteins: implications for the evolution of chordate muscle gene regulation

Developmental gene activities in ascidian embryos

The fertilized egg of ascidians develops quickly into a tadpole-type larva consisting of several distinct types of tissues including epidermis, central nervous system, endoderm, mesenchyme, notochord, and muscle. This architecture of the ascidian larva represents the most simplified chordate body plan. Taking advantage of simple, well-defined cell lineages, the expression of developmental genes is analyzed at single-cell level. Advances in the methodology promote the ascidian embryo as a useful system for studying transcriptional control involved in the specification of embryonic cells and pattern formation of the embryo.

Mutations affecting tail and notochord development in the ascidian Ciona savignyi

Ascidians are among the most distant chordate relatives of the vertebrates. However, ascidians share many features with vertebrates including a notochord and hollow dorsal nerve cord. A screen for N-ethyl-N-nitrosourea (ENU)-induced mutations affecting early development in the ascidian Ciona savignyi resulted in the isolation of a number of mutants including the complementing notochord mutants chongmague and chobi. In chongmague embryos the notochord fails to develop, and the notochord cells instead adopt a mesenchyme-like fate. The failure of notochord development in chongmague embryos results in a severe truncation of tail, although development of the tail muscles and caudal nerve tracts appears largely normal. Chobi embryos also have a truncation of the tail stemming from a disruption of the notochord. However, in chobi embryos the early development of the notochord appears normal and defects occur later as the notochord attempts to extend and direct elongation of the tail. We find in chobi tailbud embryos that the notochord is often bent, with cells clumped together, rather than extended as a column. These results provide new information on the function and development of the ascidian notochord. In addition, the results demonstrate how the unique features of ascidians can be used in genetic analysis of morphogenesis.

An alternatively spliced gene encoding a Y-box protein showing maternal expression and tissue-specific zygotic expression in the ascidian embryo

An ascidian Y-box protein gene was cloned, designated as CiYB, which consists of a highly conserved cold shock domain and an auxiliary tail domain with alternating modules of acidic and basic amino acids. CiYB is a single copy gene in the ascidian genome. During oogenesis and early development, CiYB produces three different transcripts (CiYB1, CiYB2 and CiYB3) by alternate splicing. CiYB1 and CiYB2 were expressed during oogenesis, suggesting that they are recruited into maternal ribonucleoprotein particles. According to gel mobility shift assay, the CiYB1 protein has the ability to bind RNA. The sequence preference of RNA binding is similar to that of the Xenopus Y-box protein (FRGY2), which is a major component of the maternal messenger ribonucleoprotein particles (mRNP) in the oocyte. These results suggest that the ascidian Y-box protein may have an important role for masking and translational regulation of maternal mRNA. Furthermore, CiYB1, CiYB2 and CiYB3 were expressed zygotically in a tissue restricted manner. CiYB1 was expressed specifically in muscle precursor blastomeres and tail muscle cells suggesting its important role in muscle differentiation.

Ascidian embryogenesis and the origins of the chordate body plan

For more than a century, ascidians have been a widely used system for classic embryological studies. Ascidians possess simple, well-defined cell-lineages, compact genomes, rapid development and world-wide distribution. Transgenic DNA can be introduced into developing embryos using simple electroporation methods. The ascidian larva represents the most simplified chordate body plan and provides a useful model for studying the molecular pathways underlying the morphogenesis and differentiation of the notochord and neural tube.

Tissue-specific alternative splicing of ascidian troponin I isoforms. Redesign of a protein isoform-generating mechanism during chordate evolution

In vertebrates, troponin I (TnI) exists as shorter and longer isoforms encoded by distinct genes expressed in skeletal and cardiac muscle, respectively. We report that the protochordate ascidian Ciona intestinalis expresses a homologous set of shorter and longer TnI isoforms in body wall muscle and heart, respectively. The heart-specific segment of the ascidian longer TnI isoform shares several sequence features with vertebrate cardiac TnI but lacks the protein kinase A phosphorylation sites implicated in sympatho-adrenal control of cardiac function. In contrast with vertebrates, the ascidian longer and shorter TnI isoforms are produced from a single gene by tissue-specific alternative RNA splicing; remarkably, the molecular mechanism of TnI isoform generation has been entirely reworked during ascidian/vertebrate evolution. Because alternative splicing is the more probable chordate ancestral condition, the long/cardiac versus short/somatic muscle pattern of TnI isoforms likely existed before the occurrence of the gene duplication events that created the vertebrate TnI gene family. Thus, gene duplication was apparently not the primary engine of isoform diversity in this aspect of TnI gene family evolution; rather, it simply provided an alternative (transcriptional) means of maintaining a previously established system of isoform diversity and tissue specificity based on alternative RNA splicing.