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Department of Biological Sciences, University of North Carolina at Wilmington, 601 S. College Road, Wilmington, North Carolina 28403-5915
T. W. Johnson, Jr.
3505 Manford Drive, Durham, North Carolina 27707
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ABSTRACT |
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 TOP ABSTRACT LITERATURE CITED |
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Inadequate attention to zoosporangial discharge has led to confusion in watermold taxonomic literature. This problem is discussed in light of a specimen of Protoachlya hypogyna that manifests discharge characteristic of three watermold genera, and recommendations are made to reduce future inaccuracies.
Key words: Protoachlya Saprolegniaceae, water-mold, zoosporangial discharge
We have recovered a specimen of Protoachlya hypogyna (Coker and Pemberton) Johnson and Seymour from nonsaline soil in coastal North Carolina whose reproductive morphology focuses attention on zoosporangial discharge pattern as the hallmark feature for distinguishing among saprolegniaceous genera. Significant sexual features of this isolate are summarized in TABLE I and FIGS. 1 and 2.
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On one extreme, zoospores emerging from achlyoid sporangia of isolate 242 exhibit limited motility as evidenced by brief, slow quivering before rapid encystment. Nevertheless, their behavior must be classified as achlyoid by virtue of apical clustering of encysted zoospores. Its protoachlyoid zoosporangia vary in the number of primary spores that swim away from the apical cluster. In some cases as few as two spores swim away while from other sporangia as many as 20 do so. In all cases, however, swimming is lethargic and encystment occurs uniformly when spores are still within about three zoosporangial lengths of the exit pore. Primary spores emerging from saprolegnoid sporangia also swim lethargically, but none cluster at the discharge pore, and all soon encyst within a few sporangial-lengths of the apex. In all cases secondary planonts emerging from primary spore cysts exhibit characteristic morphology and rapid swimming.
The dramatic differences in zoospore behavior upon discharge in this isolate cannot be demonstrated adequately with traditional micrographs. We, therefore, refer the reader to digitized video clips of each discharge type that this organism manifests. These real-time images are available for viewing on the Worldwide Web by accessing http://people.uncw.edu/padgett/water_mold.htm. At the bottom of the screen the viewer should click on P_hyp242 to access a second screen containing three files that are labeled SAP (the video showing saprolegnoid discharge), ACHL (achlyoid discharge) and PROTO (protoachlyoid discharge). It should be noted that viewing these images requires Real One PlayerTM software.
Numerous investigators (e.g., Humphrey 1893
, Atkinson 1909, Höhnk 1933, Dick 1990, Daugherty et al 1998) have argued that Saprolegnia is the ancestral genus within the Saprolegniaceae from which other genera—most notably Achlya, Thraustotheca and Dictyuchus—are derived. These arguments are based both on morphology and motility of the primary zoospores and are supported by DNA sequence data. We find these reports convincing and believe it important to point out that the present isolate probably does not suggest otherwise. Indeed, it seems likely that evaluating gene sequence data from a large variety of saprolegniaceous species is the only valid tool not only for answering the ancestry question but also for correctly resolving the boundaries between taxa.
Initial observations of this specimen suggested that it was an Achlya species, but further observations demonstrated clearly that it was assignable to Protoachlya (TABLE I, FIGS. 1 and 2). Our principal interest in this isolate stems from concern that inadequate attention is paid to accurate characterization of zoospore discharge in the Saprolegniaceae before identifications or descriptions of taxa are made. As an example, Steciow (2001) described Achlya fuegiana as a new taxon from Tierra del Fuego and typified its primary sporangia as thrautothecoid or achlyoid but having "some spores mov[ing] on" from the apical cluster. Based on this description, the tax on clearly exhibited at least some degree of protoachlyoid discharge, thus might well have been assigned to the wrong genus.
During preparation of the taxonomic account in a treatise of the Saprolegniaceae ( Johnson et al 2002), we concluded that numerous taxa might have been characterized incorrectly either in terms of sexual or of asexual features. As a partial remedy to what appears to be a persistent problem in the systematics of this family, we offer these criteria that should be employed before any watermold is assigned to genus: All isolates should (i) be derived from a single spore or single hyphal tip before being characterized, (ii) be free of bacterial contamination, and (iii) be assigned to genus only after observation of discharge from a minimum of 10 primary (not secondary) zoosporangia from intact colonies that are less than 48 h old and floating freely in water (not from colonies or severed hyphae mounted under a cover slip). Zoospore discharge from primary or secondary sporangia should be viewed as characteristic (typical) only in colonies less than 48 h old. This criterion is especially relevant in cases of secondary sporangium formation and subsequent discharge. Whether discharge from such sporangia is a function of genetic control or a response to metabolite accumulation remains undiscovered. In the event that zoosporangia are formed only after prolonged incubation, culture water should be changed every 48 h until discharge is observed.
Application of the recommended culture parameters may well be standard practice among most workers in this group but are offered as suggestions that should help in promoting accuracy in watermold systematics.
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FOOTNOTES |
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1 Corresponding author. E-mail: Padgett{at}uncw.edu
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LITERATURE CITED |
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TOPABSTRACTLITERATURE CITED |
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Daughtery J, Evans TM, Skillom T, Watson LE, Money NP. 1998. Evolution of spore release mechanisms in the Saprolegniaceae (Oomycetes): evidence from Phylogenetic analysis of internal transcribed spacer sequences. Fungal Genetics and Biology 24:354–363.[Medline]
Dick MW. 1990. Phylum Oomycota. In: Margulis L, Corliss JO, Melkonian M, Chapman D, eds. Handbook of the Protoctista. Jones and Bartlett, Boston. p 661–685.
Höhnk W. 1933. Polyplanetism and zoöspore germination in the Saprolegniaceae and Pythium. Amer J Bot 20: 45–62.
Humphrey JE. 1893. The Saprolegniaceae of the United States, with special notes on other species. Trans Amer Philos Soc (N. S.) 17:63–148.
Johnson TW, Jr., Seymour RL, Padgett DE. 2002. Biology and systematics of the Saprolegniaceae. Published at http://www.uncw.edu/people/padgett (click on link to Watermold Treatise).
Steciow MM. 2001. Achlya fuegiana, a new species from Tierra del Fuego Province (Argentina). Mycologia 93: 1195–1199.
This article has been cited by other articles:
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  J. P. Hulvey, D. E. Padgett, and J. C. Bailey Species boundaries within Saprolegnia (Saprolegniales, Oomycota) based on morphological and DNA sequence data Mycologia, May 1, 2007; 99(3): 421 - 429. [Abstract] [Full Text] [PDF]
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