Editing Tunicate (section)

===Promotion of out-crossing===

''[[Ciona intestinalis]]'' (class Ascidiacea) is a hermaphrodite that releases sperm and eggs into the surrounding seawater almost simultaneously.  It is self-sterile, and thus has been used for studies on the mechanism of self-incompatibility.<ref name="pmid24878524">{{cite journal |vauthors=Sawada H, Morita M, Iwano M |title=Self/non-self recognition mechanisms in sexual reproduction: new insight into the self-incompatibility system shared by flowering plants and hermaphroditic animals |journal=Biochem. Biophys. Res. Commun. |volume=450 |issue=3 |pages=1142–8 | date=August 2014 |pmid=24878524 |doi=10.1016/j.bbrc.2014.05.099 }}</ref> Self/non-self-recognition molecules play a key role in the process of interaction between sperm and the vitelline coat of the egg.  It appears that self/non-self recognition in ascidians such as ''C. intestinalis'' is mechanistically similar to self-incompatibility systems in flowering plants.<ref name="pmid24878524" /> Self-incompatibility promotes out-crossing, and thus provides the adaptive advantage at each generation of the masking of deleterious recessive mutations (i.e. genetic complementation)<ref name=Bernstein87>{{cite book | last1=Bernstein | first1=H | last2=Hopf | first2=FA | last3=Michod | first3=RE | chapter=The Molecular Basis of the Evolution of Sex | year=1987 | title=Molecular Genetics of Development | volume=24 | pages=323–70 | pmid=3324702 | doi=10.1016/S0065-2660(08)60012-7| series=Advances in Genetics | isbn=9780120176243 }}</ref> and the avoidance of [[inbreeding depression]].

''[[Botryllus schlosseri]]'' (class Ascidiacea) is a colonial tunicate, a member of the only group of chordates that are able to reproduce both sexually and asexually. ''B. schlosseri'' is a sequential (protogynous) hermaphrodite, and in a colony, eggs are ovulated about two days before the peak of sperm emission.<ref name="pmid25044771">{{cite journal |author1=Gasparini, F |author2=Manni, L |author3=Cima, F |author4=Zaniolo, G |author5=Burighel, P |author6=Caicci, F |author7=Franchi, N |author8=Schiavon, F |author9=Rigon, F |author10=Campagna, D |author11=Ballarin, L |title=Sexual and asexual reproduction in the colonial ascidian Botryllus schlosseri |journal=Genesis |volume=53 |issue=1 |pages=105–20 | date=July 2014 |pmid=25044771 |doi=10.1002/dvg.22802 |s2cid=205772576 }}</ref> Thus self-fertilization is avoided, and cross-fertilization is favored.  Although avoided, self-fertilization is still possible in ''B. schlosseri''.  Self-fertilized eggs develop with a substantially higher frequency of anomalies during cleavage than cross-fertilized eggs (23% vs. 1.6%).<ref name="pmid25044771" /> Also a significantly lower percentage of larvae derived from self-fertilized eggs metamorphose, and the growth of the colonies derived from their metamorphosis is significantly lower.  These findings suggest that self-fertilization gives rise to inbreeding depression associated with developmental deficits that are likely caused by expression of deleterious recessive mutations.<ref name=Bernstein87 />