Taxonomy and distribution of Australian cephalochordates
Our data also provides information on the relative importance of these three groups in comparison with other main macro-invertebrates from the Punta Morales flat collected during the 1984-1987 survey, like cephalochordates
(Vargas & Dean, 2010), mollusks (Vargas-Zamora & Sibaja-Cordero, 2011), echinoderms (Vargas & Solano, 2011), crustaceans (Vargas-Zamora, Sibaja-Cordero, & Vargas-Castillo, 2012) and polychaetes (Vargas-Zamora, Sibaja-Cordero, Dean, & Solano-Ulate, 2015).
In muddy sediments the cephalochordate
remains on the sediment surface.
For the cephalochordates
considered here, the exceptionally long separation of the parent taxa--since the latter part of the Mesozoic Era--raises interesting questions about how the divergent gene regulatory networks from the two parents function together and how the hybrid genotype directs the development of the hybrid phenotype.
Although several hypotheses were postulated to explain the origin of protamines in vertebrates (see the following), there were no studies at that time on the SNBPs of other zoological groups with close affinity to the vertebrates, such as chordates or cephalochordates
, to help understand the problem.
Ascidians are members of the subphylum urochordates, which is a group of the phylum chordates along with cephalochordates
and vertebrates (Satoh, 2003).
At present, the significance of these new findings is obscured by uncertainty about whether characters in the genus Asymmetron are more likely to be primitive or derived within the cephalochordates
. It will, therefore, be important to resolve the main course of cephalochordate
evolution by further phylogenetic analyses with nuclear genes and by sequencing the genome from a species of Asymmetron.
Recent molecular phylogenetic studies have indicaed that cephalochordates
diverged first from the chordate stem (Delsuc et al., 2006; Bourlat et al., 2006; Putnam et al., 2008), but it is believed that they retain ancestral characters more than the other chordate groups (Putnam et al., 2008).
1 A), and both phyla make up the sister group of the chordate phylum, which includes urochordates, cephalochordates
, and vertebrates (Castresana et al., 1998; Bromham and Degnan, 1999; Cameron et al., 2000; Furlong and Holland, 2002; Smith et al., 2004; Bourlat et al., 2006).
In contrast to urochordates, cephalochordates
have retained many features of the basal chordate body plan and are likely to approximate the ancestor of both urochordates and vertebrates.