Towards and Understanding of the Evolution of the Downy Mildews

Abstract

PeronosporalesObligate parasites of dicotyledons (very rarely monocotyledons). Thallus mycelial and intercellular with haustoria; zoosporogenesis, when present, by internal cleavage, otherwise asexual reproduction by deciduous conidiosporangia; conidiosporangiophores well-differentiated, persistent; oogonia thin-walled, oospore single, aplerotic with a well-defined exospore wall layer derived from persistent periplasm. Possibility that all are dependent on exogenous sources of sterols. Peronosporaceae: Myceliar fungi with large, lobate haustoria. Asexual reproduction by deciduous PythialesParasites or saprotrophs; parasites mostly in axenic culture. Some members parasitic on fungi and some on animals. Thallus mycelial with little evidence of cytoplasmic streaming; zoosporangium formation terminal, less frequently sequential, then percurrent or by internal or sympodial proliferation; sporangiophores rarely differentiated; oogonial periplasm minimal and not persistent; oospore usually single, plerotic or aplerotic. Evidence of partial dependence on exogenous sterol precursors. Pythiaceae: Thallus mycelial or monocentric and pseudomycelial; hyphae diameter;conidiosporangia or conidia borne on conidiosporangiophores; conidiosporangia pedicellate; conidiosporangiophores dichotomously branched, monopodially branched, or unbranched and clavate; conidiogenesis simultaneous; zoosporogenesis, when present, internal within a plasmamembranic membrane, zoospore release by operculate or poroid discharge. deciduous sporangia, conidiosporangia or conidia borne on unbranched conidiophores. Conidiogenesis sequential and percurrent. Zoosporogenesis internal with papillate discharge. Albuginaceae:Myceliar fungi with small, spherical or peg-like haustoria. Asexual reproduction by Genus: Albugo.zoosporogenesis either by internal cleavage without vesicular discharge or with a plasmamembranic vesicle or by external cleavage in a homohylic vesicle; oogonia (with very few exceptions) thin-walled; oospores never strictly plerotic. Aerobic metabolism. Freshwater or marine.

Evidence of parasitic Oomycetes (Peronosporomycetes) infecting the stem cortex of the Carboniferous seed fernLyginopteris oldhamia

“…Bhattacharya et al [17] suggested a mean age for the split between Oomycetes and Bacillariophyta of 936 Ma (range: 1150 -770 Ma). Internal calibration of the Oomycetes molecular phylogeny has not yet been attempted, but Dick [1,18] presented a compelling case for a comparatively recent (Cenozoic Era) radiation of the phylogenetically derived Peronosporales (Downy Mildews). Fossil evidence prior to the Cretaceous Period is rare, and where fossils have been documented their affinities are difficult to establish [19].…”

The Compatible Interaction in Downy Mildew Infections

“…It should be remembered, however, that downy mildews do not always form haustoria (Fraymouth, 1956), and their intercellular hyphae also provide an interface for nutrient uptake and general inter-communication, as in other non-haustorial biotrophs (Spencer-Phillips, 1997). Indeed Dick (2002a) has pointed out that the development of haustoria may have evolved separately even within the oomycetes, so that haustoria may have a different physiology and function in each of the genera Albugo, Peronospora and Sclerospora.…”

“…Downy mildews are oomycetes belonging to the orders Peronosporales and Sclerosporales in the class Peronosporomycetes, within the kingdom Straminipila, according to Dick (2002aDick ( , 2002b. The taxonomy of this group of organisms, however, is under continuous revision.…”

“…Although these oomycetes do not map phylogenetically to the kingdom Fungi, they are considered to be fungi in the broadest sense because they share a life-style based on a hyphal and mycelial mode of growth, an absorptive heterotrophic nutrition and reproduction via spores (Dick, 2002a;Diéguez-Uribeondo et al, 2004). The downy mildew fungi are also examples of endophytic biotrophs forming an intercellular mycelium, typically with intracellular haustoria, and gaining nutrients from living host cells.…”

“…A recent review of resistant interactions and the mechanisms of resistance is provided by Mauch-Mani (2002), whilst evolutionary aspects together with a critical analysis of the key features of the downy mildew pathogens are considered by Dick (2002a). Detailed descriptions of symptoms, the formation, survival and germination of asexual spores and oospores, and various other aspects of the biology of individual downy mildew pathogens are given in chapters in Spencer (1981), the IMI descriptions published by CAB International Mycological Institute (Kew, Surrey, UK), and the Compendium of Plant Diseases series published by the American Phytopathological Society (eg see Biddle, 2001).…”