Colony specificity in marine invertebrates and contact reaction in ascidian hemocytes
Preface
Edited by Euichi Hirose
Self or non-self discrimination is one of the fundamental functions for animals to maintain individuality, and materials recognized as non-self are usually decomposed and/or eliminated from the body. Many animals are also able to discriminate self and non-self cells/tissues within the same species (allorecognition). This recognition system is possibly involved in the recognition and elimination of tumor cells. In metazoans, although allorecognition is often represented by graft rejection in tissue transplantation, this is an artificially induced response. Natural occurrence of cellular allorecognition can be found in mother–fetus interactions in viviparous animals, self-sterility in fertilization of hermaphrodite animals, and colonial fusion in colonial animals. This virtual issue mainly deals with the last topic, so-called colony specificity.
Colony specificity is an occurrence of allorecognition when conspecific colonies come into contact. The colonies either fuse into one colony or reject the other contacting colony, depending on the combination of the colonies. Colonies recognize the partner as self in the former case and recognize it as non-self in the latter case. Whereas the occurrence of colony specificity has been reported from various taxa from poriferans to chordates, colonial animals do not always have the capacity of the colonial allorecognition. In species that do not show colony specificity, even syngeneic colonies originating from the same clone colony always ignore one another and never fuse into one colony. Colony specificity has been well studied in colonial ascidians, particularly in the species of the family Botryllidae (i.e., botryllid ascidians), because the sheet-like colonies grow fast and are convenient for handling. Articles on ascidian colony specificity are listed in Chapter 1. When a botryllid colony is brought into contact with non-self colonies, a rejection reaction is induced around the border between the two colonies. The effectors of the rejection reaction are hemocytes and tunic cells. Chapter 2 covers articles on the classification and functions of botryllid hemocytes and tunic cells. Articles on colony specificity in other marine invertebrates are listed in Chapter 3.
Colony specificity is namely unique to colonial organisms. In the solitary ascidian Halocynthia roretzi, mixed-incubation of hemocytes from allogeneic individuals induces a hemolytic reaction, i.e. a contact reaction, depending on the combinations of individuals. Chapter 4 covers studies on contact reactions in H. roretzi hemocytes. Needless to say, hemocytes play the leading role in contact reactions. However, there has been confusion regarding hemocyte classification in H. roretzi, as several research groups have named hemocyte types independently. Chapter 5 covers articles on hemocyte classification and immunological functions in H. roretzi.