Amoebidiidae is a family of single-celled , previously thought to be zygomycete fungi belonging to the class Trichomycetes, but molecular phylogenetic analyses[Benny, G. L., and O'Donnell, K. 2000. Amoebidium parasiticum is a protozoan, not a Trichomycete. Mycologia 92: 1133-1137.][Ustinova, I, Krienitz, L., and Huss, V. A. R. 2000. Hyaloraphidium curvatum is not a green alga, but a lower fungus; Amoebidium parasiticum is not a fungus, but a member of the DRIPS. Protist 151: 253-262.][Cafaro, M. 2005. Eccrinales (Trichomycetes) are not fungi, but a clade of protists at the early divergence of animals and fungi. Molecular Phylogenetics and Evolution 35: 21-34.] place the family with the opisthokont group Mesomycetozoea[Mendoza L, Taylor JW, Ajello L (October 2002). "The class mesomycetozoea: a heterogeneous group of microorganisms at the animal-fungal boundary". Annu. Rev. Microbiol. 56: 315–44. doi:] (= Ichthyosporea[Cavalier-Smith, T. 1998. Neomonada and the origin of animals and fungi. In: Coombs GH, Vickerman K, Sleigh MA, Warren A (ed.) Evolutionary relationships among protozoa. Kluwer, London, pp. 375-407.]). The family was originally called Amoebidiaceae,[
]
/ref> Species of Amoebidium are considered obligate Symbiosis of freshwater-dwelling arthropod hosts such as Chironomidae and water fleas ( Daphnia).[Lichtwardt, R.W. 2001. Trichomycetes: fungi in relationship with insects and other arthropods. In: Symbiosis. J. Seckbach, ed. Kluwer Academic Publishers, Netherlands, p. 515-588.] However, because Amoebidium species attach to the exoskeleton (exterior) of the host and grow in axenic, at least some species may be facultative symbionts.[Lichtwardt, R.W., M.J. Cafaro, M.M. White. 2001. The Trichomycetes: Fungal Associates of Arthropods Revised Edition. Published online http://www.nhm.ku.edu/%7Efungi/Monograph/Text/Mono.htm ]
Etymology
The "amoeb-" prefix refers to the amoeba-like dispersal cells that are produced during the life cycle of Amoebidium.[Lichtenstein, J. L. 1917a. Sur un Amoebidium a commensalisme interne du rectum des larves d Anax imperator Leach: Amoebidium fasciculatum n. sp. Archives de Zoologie Expérimentale et Générale 56': 49-62.]
Description
As the family is monogeneric, the description follows that of the genus, Amoebidium. Amoebidium species are single-celled, cigar-shaped or tubular in vegetative growth form (= thallus), and attach to the exoskeleton of various freshwater arthropod hosts (Crustacean or ) by means of a secreted, glue-like basal holdfast.
Taxonomic and phylogenetic history
The classification of Amoebidium and Paramoebidium with the fungal trichomycetes was early considered tenuous due to the production of amoeboid dispersal cells,[Léger, L., and Duboscq, O. 1929. L'évolution des Paramoebidium, nouveau genre d'Eccrinides, parasite des larves aquatiques d'Insectes. Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences Paris 189: 75-77.][Cienkowski, L. 1861. Ueber parasitische Schläuche auf Crustaceen und einigen Insektenlarven ( Amoebidium parasiticum m.). Botanische Zeitung 19: 169-174.] a feature not seen among Fungi. Later studies found no evidence of chitin in the cell wall of these species, further casting doubt on their relatedness to Fungi.[Whisler, H.C., 1963. Observations on some new and unusual enterophilous phycomycetes. Canadian Journal of Botany, 41(6), pp.887–900.][Trotter, M.J. & Whisler, H.C., 1965. Chemical composition of the cell wall of Amoebidium parasiticum. Canadian Journal of Botany, (43), pp.869–876.] However, their overall morphology (i.e. hair-like growth form with a basal holdfast), production of spores, and residence in the digestive tract of arthropods were considered strong enough characters to include them with the fungal trichomycetes until additional evidence could resolve their placement.[Moss, S.T., 1979. Commensalism of Trichomycetes. In L. R. Batra, ed. Insect-Fungus Symbiosis Nutrition, Mutualism, and Commensalism. Montclair: Allanheld, Osmun & Co. Publishers, Inc., pp. 175–227.]
In 2000, two independent studies[Ragan MA, Goggin CL, Cawthorn RJ, et al. (October 1996). "A novel clade of protistan parasites near the animal-fungal divergence". Proc. Natl. Acad. Sci. U.S.A. 93 (21): 11907–12. doi:] at the time) than the fungal trichomycetes. In 2005, Cafaro
However, a recent molecular phylogenetic analysis of the Ichthyophonida that included broad taxon and gene sampling found evidence of polyphyly between Amoebidium and Paramoebidium.[Shimodaira, H., 2002. An approximately unbiased test of phylogenetic tree selection. Syst. Biol. 51, 492–508.] and Shimodaira-Hasegawa[Shimodaira, H., Hasegawa, M., 1999. Multiple Comparisons of Log-Likelihoods with Applications to Phylogenetic Inference. Mol. Biol. Evol. 16, 1114–1116.] tests) did not reject a sister-taxon relationship of the genera, the authors felt there was enough molecular, physiological, and ecological differences to separate the genera into different families.[Dang, S.-N., Lichtwardt, R.W., 1979. Fine Structure of Paramoebidium (Trichomycetes) and a new species with virus-like particles. Am. J. Bot. 66, 1093-1104.]
