"Yeast" is, admittedly, not one of the most tasteful-sounding words in the English language (it's right up there with "moist" and "sphagnum" in the category of words that sound crude without necessarily meaning anything offensive) but yeasts are an abundant component of our environment. They are most familiar as a vital component in making many foods (such as the baker's or brewer's yeast, Saccharomyces cerevisiae, in bread and beer) or less welcomely as opportunistic pathogens (such as Candida albicans, the cause of thrush). Technically speaking, yeasts are fungi that have abandoned the hyphal growth-form of most fungi and reverted to a more unicellular mode of life. Phylogenetically, yeasts are scattered throughout the fungal family tree, and it is this scattered distribution among more standard hyphal fungi that suggests that the yeast form has arisen multiple times independently.
The largest grouping of yeasts is the class Saccharomycetes. While morphologically fairly uniform (lets face it, there's only so much you can do with a relatively unornamented unicell), Saccharomycetes show a reasonable degree of diversity in biochemistry. The most recent (2006) edition of the Outline of Ascomycota (Eriksson, 2006 - the kind-of-sort-of official classification of ascomycete fungi) places all Saccharomycetes in a single order, the Saccharomycetales, but I personally prefer the admittedly less popular idea of dividing Saccharomycetes into a number of orders (as was done, for instance, by Schweigkofler et al., 2002) as I feel that this (a) better reflects said chemical diversity, and (b) reduces the redundant uninformativeness of having only a single order in a single class.
Within the Saccharomycetales (however you want to define it), the genus Metschnikowia contains spherical to cylindrical yeasts that reproduce by multilateral budding and produce elongate to club-shaped asci containing one or two needle-shaped spores each (Miller et al., 1967 - if you're wondering what an ascus is, feel free to look it up on Palaeos). Successive rounds of budding may lead to the formation of a pseudomycelium (the image at the top of the page shows Metschnikowia pulcherrima and comes from the INRA). The presence of only two instead of the expected four spores per ascus is due to the spores originating after meiosis stage I (segregation of homologous chromosomes) but before meiosis II (separation of sister chromatids). Meiosis II occurs as per normal after spore division, but one of the resultant nuclei is degraded in each cell (Marinoni et al., 2003). However, I must confess ignorance of what happens in those species with only a single spore per ascus.
Metschnikowia was first discovered as a parasite of water fleas (Daphnia magna, small freshwater crustaceans) but various species and their Candida anamorphs* have since been found in association with a wide range of arthropods and plants (sometimes both - Candida kunwiensis, the anamorph of Metschnikowia kunwiensis - Brysch-Herzberg, 2004 - was isolated in a Korean laboratory from flowers of Ipomoea batatas (kumara) at the same time as German researchers independently isolated it from the bodies of pollinating bumblebees - Hong et al., 2003). Most are probably commensals on their hosts as opposed to parasites.
*An "anamorph" is the asexually reproducing form of a fungus, as opposed to the sexually reproducing teleomorph. Someday I shall write about the complicated and confusing system of double nomenclature attendant on fungi whereby asexual forms are classified separately from sexual forms, and the combination of history, theory and a certain degree of pragmatism that has lead to this system. Today, however, is not that day.
Recently certain Metschnikowia species have gained a bit of attention as potential biocontrol agents, as plant-associated forms have been demonstrated to inhibit the growth of other micro-organisms on their hosts. Metschnikowia yeasts may be very useful in inhibiting the growth of mould on harvested fruit, and at least one patent exists suggesting coating harvested fruit in a mixture containing isolated Metschnikowia culture.
Brysch-Herzberg, M. 2004. Metschnikowia kunwiensis comb. nov., the teleomorph of Candida kunwiensis. FEMS Yeast Research 4: 605-607.
Eriksson, O. E. (ed.) 2006. Outline of Ascomycota - 2006. Myconet 12: 1-82.
Hong, S. G., K. S. Bae, M. Herzberg, A. Titze & M.-A. Lachance. 2003. Candida kunwiensis sp. nov., a yeast associated with flowers and bumblebees. International Journal of Systematic and Evolutionary Microbiology 53: 367-372.
Marinoni, G., J. Piskur & M. A. Lachance. 2003. Ascospores of large-spored Metschnikowia species are genuine meiotic products of these yeasts. FEMS Yeast Research 3 (1): 85-90.
Miller, M. W., E. R. Barker & J. I. Pitt. 1967. Ascospore numbers in Metschnikowia. Journal of Bacteriology 94 (1): 258-259.
Schweigkofler, W., K. Lopandic, O. Molnár & H. Prillinger. 2002. Analysis of phylogenetic relationships among Ascomycota with yeast phases using ribosomal DNA sequences and cell wall sugars. Organisms, Diversity and Evolution 2: 1-17.