Adeleorina Leger 1977

Note: this tree is still under construction. It does not yet contain all known Adeleorina subgroups.

Introduction

Suborder Adeleorina Léger, 1911

Two groups belong to this suborder: (i) “adelines” – monoxenous coccidians of invertebrates, and (ii) “haemogregarines” – heteroxenous coccidians cycling between blood-sucking invertebrates (definitive hosts) and various vertebrates (intermediate hosts). The genus Klossiella (Klossiellidae), a monoxenous coccidium of mammals and reptiles, represents an exception. In veterinary medicine, the most important member representing this group is the genus Hepatozoon, whose member species are mainly pathogenic in domestic mammals such as canids.

Characteristics

Adeleorina are coccidia with the syzygy method of gamete formation, involving the association of often motile gamonts prior to the formation of functional gametes and fertilization. In haemogregarines with heteroxenous species, conjugation of gamonts and subsequent sporogony usually occurs within an invertebrate definitive host which serves as the vector. Similar to all members of the Eucoccidiorida, there is a complex life cycle involving at least one (and often numerous) asexual cycles of merogony followed by gametogony, syngamy, and sporogony. Many species of the group have morphologically distinct types of meronts and merozoites during asexual replication: the first type of meront produces large merozoites which are believed by many to start a further round of merogonic replication; the second type of meront produces smaller merozoites which are said to be progenitors of the gamonts.

The Hemogregarines are important in the history of parasitology. The first species to be seen was apparently Dactylosoma ranarum, by Lankester (1871) in a frog in Europe; he called it Undulina ranarum but later (1882) changed the name to Drepanidium ranarum. So far, there are ~500 named species, almost certainly a great underestimation of the real diversity. Members of the genera Adelina and Adelea infect mostly insects, whereas Klossia is a model coccidium from molluscs.

The haemogregarines (Hepatozoidae, Haemogregarinidae, and Dactylosomatidae) comprise several genera, including pathogens of vertebrates, such as Hepatozoon from carnivores and reptiles, and Haemogregarina from fish and turtles. For all these species, invertebrates play the role of the definitive host with gamogony in their digestive system. Then, basically two modes of transmission occur: (i) the inoculative way (Haemogregarina, Dactylosoma), when the infectious sporozoites enter the vertebrate host during blood-feeding of the vector; (ii) alternatively, the parasite is transmitted by the ingestion of the infected definitive (=invertebrate) host by the appropriate vertebrate host (Hepatozoon, Haemolivia, Karyolyssus). The latter mode of transmission may even involve a paratenic host. Regardless of the mode of transmission, the merogonial division of hemogregarines usually takes place in the parenchymatous organs of vertebrates, followed by the formation of infective gametocytes in the circulating red (in the case of Hepatozoon also white) blood cells. The next definitive host is infected exclusively through blood feeding.

Discussion of Phylogenetic Relationships

Phylogenetic studies indicate that both groups, the monoxenous and heteroxenous coccidians, form a monophyletic clade, characterized also by several morphological and developmental features. Microgamonts produce usually only one to four microgametes, which associate with the macrogamete in syzygy. Other characteristic features of Adeleorina are the absence of endodyogony and the enclosure of sporozoites in a sporocyst.