Morphological Differences between Larvae of the Ciona intestinalis Species Complex: Hints for a Valid Taxonomic Definition of Distinct Species

Abstract

The cosmopolitan ascidian Ciona intestinalis is the most common model species of Tunicata, the sister-group of Vertebrata, and widely used in developmental biology, genomics and evolutionary studies. Recently, molecular studies suggested the presence of cryptic species hidden within the C. intestinalis species, namely C. intestinalis type A and type B. So far, no substantial morphological differences have been identified between individuals belonging to the two types. Here we present morphometric, immunohistochemical, and histological analyses, as well as 3-D reconstructions, of late larvae obtained by cross-fertilization experiments of molecularly determined type A and type B adults, sampled in different seasons and in four different localities. Our data point to quantitative and qualitative differences in the trunk shape of larvae belonging to the two types. In particular, type B larvae exhibit a longer pre-oral lobe, longer and relatively narrower total body length, and a shorter ocellus-tail distance than type A larvae. All these differences were found to be statistically significant in a Discriminant Analysis. Depending on the number of analyzed parameters, the obtained discriminant function was able to correctly classify > 93% of the larvae, with the remaining misclassified larvae attributable to the existence of intra-type seasonal variability. No larval differences were observed at the level of histology and immunohistochemical localization of peripheral sensory neurons. We conclude that type A and type B are two distinct species that can be distinguished on the basis of larval morphology and molecular data. Since the identified larval differences appear to be valid diagnostic characters, we suggest to raise both types to the rank of species and to assign them distinct names.

Characterization of Ambra1 in asexual cycle of a non-vertebrate chordate, the colonial tunicate Botryllus schlosseri, and phylogenetic analysis of the protein group in Bilateria

“…Before this study, expression of Ambra1, together with expression of other genes involved in the mammalian autophagic process but not present in yeast, was demonstrated only in the tunicate C. robusta (Godefroy et al, 2009), formerly Ciona intestinalis type A (see Brunetti et al, 2015;Pennati et al, 2015). In the latter species, Ambra1 expression, as analyzed by means of semiquantitative RT-PCR, is developmentally regulated (Godefroy et al, 2009).…”

“…They are likely present in human and mouse, as suggested by the occurrence of different Ambra1 protein isoforms in the UniProt website (http:// www.uniprot.org/uniprot/) (Bateman et al, 2015) and by preliminary bioinformatics analysis of databases containing human and murine cDNA sequences. Looking for tunicate alternative splicing in this region, we repeated the 3 0 -RACE analysis three times and also analyzed B. schlosseri, C. intestinalis, formerly Ciona intestinalis type B (see Brunetti et al, 2015;Pennati et al, 2015) and C. robusta EST libraries without finding longer transcripts. Analysis of genomic sequences flanking to the Ambra1 locus in C. intestinalis and C. robusta did not reveal any possible coding region that may encode putative C-terminal peptide of longer form of Ambra1 protein.…”

“…In the tunicate Ciona robusta, formerly Ciona intestinalis type A (see Brunetti et al, 2015;Pennati et al, 2015), a genome search identified the Ambra1 gene, together with other genes involved in the autophagic process (Godefroy et al, 2009). These genes were compared to their homologues in S. cerevisiae, C. elegans, D. melanogaster and H. sapiens (Godefroy et al, 2009).…”

Seasonal patterns of settlement and growth of introduced and native ascidians in bivalve cultures in the Ebro Delta (NE Iberian Peninsula)

“…The cosmopolitan taxon Ciona 149 intestinalis was recently shown to comprise several cryptic species, of which the most 150 widespread are the so-called Ciona intestinalis type A and type B (Caputi et al 2007; Zahn 151 et al 2010). Recent work ; Pennati et al 2015) has shown that C. 152 intestinalis type A is in fact C. robusta, a species present in both sides of the Pacific, the 153 Indian Ocean, the English Channel, and the Mediterranean. Recent genetic data 154 (Bouchemousse et al 2016a) support the introduced status of C. robusta in Europe.…”

ACAM, a novel member of the neural IgCAM family, mediates anterior neural tube closure in a primitive chordate

“…Ciona intestinalis type A [also known as Ciona robusta (Pennati et al, 2015)] were collected in the Santa Barbara Yacht Harbor.…”