Evolution & Systematics

Tunicates represent the most basal of the chordates however it is debated if this should be the case or if tunicata might be its own phylum instead of being a sub-phylum within chordata, depending on what method of DNA analysis is used (Fig. 10. Rychel et.al 2005;Zeng et.al 2006; Singh et.al 2009).  This is despite the fact that they displaythe five characteristics of a chordate in their larval stage (Fig. 11);

1.      Dorsal nerve tube

2.      Notochord

3.      Post anal tail

4.      Endostyle

5.      Pharyngeal slits

Regardless of their relation to chordates the lineage of the tunicates extends to Precambrian where it is thought that the last common ancestor had evolved (Rychel et.al2005).  Since then they ascidiacea have diverged into orders; Aplousobranchiata, Stolidobranchia, Thaliacea and Phlebobranchiata.  P.papillatais in the order Stolidobranchia.

Stolidobranchia has both solitary and colonal orders but it has been demonstrated that colonal morphologies have evolved multiple times independent of each other (Zeng et.al2006).  As colonal morphologies and life histories have evolved within Stolidobranchia it indicates that the ancestral form is solitary (Zeng et.al 2006).  This indicates that P.papillata retains the ancestral condition of the solitary body form. 

Stolidobranchia splits into three families; Styelidae, Pyuridae and Molguidae, it is generally written that all the families within Stolidobranchia diverged at the same time (Fig. 12).  Although recent molecular data indicated that Styelidae might be paraphyletic (Pérez-Portela et.al 2009).  With P.papillata within Styelidae this complicates the evolutionary history.  It has also been demonstrated that Styelidae has had multiple evolutions of coloniality which is further complicated by the findings that based on molecular data Polycarpa seems to be polyphyletic (Pérez-Portela et.al2009).  

With the evolution of Styelidae still heavily debated and not fully understood the exact position of P.papillata on the evolutionary tree is variable. The general phylogeny can be seen below.