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Distribution and ecology of dictyostelid cellular slime molds in Great Smoky Mountains National Park

     Department of Biology, Shepherd University, Shepherdstown, West Virginia 25443

Steven L. Stephenson

     Department of Biological Sciences, University of Arkansas, Fayetteville, Arkansas 72701

James C. Cavender

     Department of Environmental and Plant Biology, Ohio University, Athens, Ohio 45701

	ABSTRACT

TOP ABSTRACT INTRODUCTION THE STUDY AREA MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 

Great Smoky Mountains National Park encompasses an area of 2080 km² in eastern Tennessee and western North Carolina between 35°28' and 35°47'N. Elevations are 270–2000 m above sea level, and the topography and vegetation are as diverse as any region of eastern North America. In 1998–2004 soil/litter samples for isolation of dictyostelid cellular slime molds were collected throughout the park. Collecting sites included examples of all major forest types along with the more common types of nonforest vegetation. More than 2300 clones of dictyostelids were recovered from 412 samples. These clones included representatives of 20 described species together with at least 10 species new to science. This total is higher than those reported for other temperate regions of the world. In general both numbers of species and numbers of clones/g of sample material decreased with increasing elevation and several species displayed a distinct preference for either the low or high end of the elevation gradient. The relatively high number of new species recovered from samples collected at high elevations is an important new finding for dictyostelid ecology and distribution.

Key words: ATBI, dictyostelids, ecology, forests, soils, southern Appalachians

	INTRODUCTION

TOP ABSTRACT INTRODUCTION THE STUDY AREA MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Dictyostelid cellular slime molds are single-celled, eukaryotic, phagotrophic bacterivores usually present and often abundant in terrestrial ecosystems (Raper 1984). These organisms represent a normal component of the microflora in soils and apparently play a role in maintaining the natural balance that exists between bacteria and other microorganisms in the soil environment. For most of their lives dictyostelids exist as separate, independent, amoeboid cells (myxamoebae) that feed on bacteria, grow and multiply by binary fission. When the food supply within a given microsite becomes depleted, numerous myxamoebae aggregate to form a structure called a pseudoplasmodium, within which each cell maintains its individual integrity. The pseudoplasmodium then produces one or more fruiting bodies (sporocarps) bearing spores. Dictyostelid fruiting bodies are microscopic and rarely observed except in laboratory culture. Under favorable conditions the spores germinate to release myxamoebae and the life cycle begins anew. In those species with spores characterized by polar granules (PG+), myxamoebae can quickly halt development and form resistant microcysts. Even more resistant macrocysts are produced as a result of sexuality in many dictyostelids, both PG+ and PG– (spores without polar granules) species. Dictyostelids are most abundant in the surface humus layer of forest soils, where populations of bacteria are the highest and microenvironmental conditions appear to be the most suitable for dictyostelid growth and development (Raper 1984).

Approximately 100 species of dictyostelids have been described formally. Some species appear to be cosmopolitan, whereas others have a more restricted distribution (Swanson et al 1999). The primary objective of the present study, which was carried out in the context of the All Taxa Biodiversity Inventory (ATBI) project taking place in Great Smoky Mountains National Park, was to determine just what species of dictyostelids were present in the southern Appalachians. The topography and vegetation of the park are as diverse as any region of eastern North America, which would seem to make it an especially appropriate place to study these organisms because some data suggest that dictyostelid diversity corresponds to higher plant diversity (Cavender et al 2004). A secondary objective was to assess the ecology and general distribution patterns of the dictyostelids recorded during the study.

	THE STUDY AREA

TOP ABSTRACT INTRODUCTION THE STUDY AREA MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Great Smoky Mountains National Park encompasses an area of 2080 km² in eastern Tennessee and western North Carolina between 35°28' and 35°47'N. Elevations are approximately 270–2000 m above sea level. Annual precipitation varies from about 140 cm at low elevations to more than 220 cm for the highest elevations. Five forest types are dominant over most of the park, with other types of communities (e.g. shrub balds [i.e. treeless areas], grassy balds, bogs, old fields and rock outcrop communities) having a more limited distribution. Red spruce (Picea rubens Sarg.)-Fraser fir (Abies fraseri [Pursh] Poiret) forests are found at elevations above 1525 m, and northern hardwood forests occur at middle elevations (1065–1525 m). At lower elevations (generally below 1065 m), pine (Pinus spp.)-oak (Quercus spp.) forests occupy drier sites and hemlock (Tsuga canadensis [L.] Carr.) forests often occur along riverbanks. Cove hardwood forests, the most diverse of all the forest types, are found in valleys throughout the park. Among the more important and widely distributed trees in these forests are tulip tree (Liridodendron tulipifera L.), beech (Fagus grandifolia Ehrhart), and sugar maple (Acer saccharum Marshall). More detailed information on all of these forest types is provided in Whittaker (1956) and Stephenson et al (2001).

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION THE STUDY AREA MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
In 1993–2004, 412 samples for isolation of dictyostelids were collected from 25 study sites throughout the park. These included examples of all of the major forest types described in the previous section along with the more common types of nonforest vegetation. A brief description of each of the primary study sites is given below. Sites are listed in order of decreasing elevation.

Sites in Great Smoky Mountains National Park from which samples were collected: (i) Clingmans Dome (CD): 35°33'40''N, 83°29'49''W, elevation 1920 m, red spruce-Fraser fir forest (near the ATBI plot); (ii) Indian Gap (IG): 35°35'11''N, 83°28'22''W, elevation 1878 m, beech forest; (iii) Andrews Bald (AB): 35°32'20''N, 83°29'39''W, elevation 1719 m, boggy area of a grassy bald dominated by Danthonia compressa Austin, with some scattered red spruce; (iv) Double Springs Gap (DS): 35°33'57''N, 83°32'28''W, elevation 1676 m, beech-yellow birch (Betula lutea Michaux f.)-buckeye (Aesculus octandra Marshall) forest; (v) Balsam Mountain (BM): 35°34'25''N, 83°10'47''W, elevation 1646 m, red spruce-hemlock-Fraser fir forest; (vi) Noland Divide Trail (ND): 35°33'36''N, 83°28'43''W, elevation 1615 m, northern hardwood forest; (vii) Indian Gap Trail (IGT): 35°36'37''N, 83°26'50''W, elevation 1570 m, northern hardwood forest with an admixture of red spruce; (viii) Bunches Bald (BB): 35°31'04''N, 83°11'38''W, elevation 1525 m, northern hardwood forest with an admixture of red spruce; (ix) Purchase Knob (PK): 35°35'17''N, 83°03'54''W, elevation 1470 m, northern hardwood forest; (x) Snakeden Ridge (SD): 35°44'33''N, 83°12'57''W, elevation 915 m, mixed hardwoods forest; (xi) Ramsey Cascade (RC): 35°42'26''N, 83°20'32''W, elevation 900 m, xeric oak forest with chestnut oak (Quercus prinus L.), scarlet oak (Q. coccinea Muenchh.), black oak (Q. velutina Lam.) the most important species present; (xii) Chimneys (CH): 35°38'24''N, 83°29'49''W, elevation 850 m, cove hardwood forest near the Chimneys campground and along the road from Sugarlands to Newfound Gap; (xiii) Western Foothills Parkway (WP): 35°38'16''N, 83°56'12''W, elevation 760 m, xeric oak-pine forest; (xiv) Maddron Bald Trail (MB): 35°45'08''N, 83°16'32''W, elevation 760 m, cove hardwoods forest about 0.8 km N of the Albright Grove Loop Trail; (xv) Poplar Hemlock (PH): 35'40''N; 83'35''W, elevation 750 m, tulip tree-hemlock-mixed hardwoods forest near the junction of the Sugarlands Mountain Trail and Little River Road; (xvi) Rich Mountain (RM): 35°38'42''N; 83°48'21''W, elevation ca. 750 m, mixed hardwoods forest near the mouth of Bull and Calf caves on Rich Mountain; (xvii) Fontana Dam (FD): 35°27'52''N, 83°48'49''W, elevation 730 m, mixed oak-pine-red maple (Acer rubrum L.)-tulip tree forest; (xviii) Ravensford (RA): 35°30'24''N, 83°17'17''W, elevation 615 m, wetland area near Oconoluftee; (xix) Oconoluftee (OC): 35°30'29''N, 83°18'11''W, elevation 610 m, alluvial floodplain forest; (xx) Twin Creeks (TC): 35°41'09''N, 83°29'58''W, elevation 600 m, mixed hardwoods forest; (xxi) Deep Creek Bog (DC): 35°28'55''N, 83°25'30''W, elevation 595 m, wetland surrounded by a pine-red maple-tulip tree forest; (xxii) Gregory’s Cave (GF and GC): 35°36'36''N, 83°48'28''W, elevation 585 m, limestone cave in a pine-mixed hardwoods forest (GF for samples from forest outside the cave and GC for samples collected within the cave); (xxiii) Eastern Foothills Parkway (EP): 35°48'26''N, 83°14'09''W, elevation 565 m, xeric oak-pine-mixed hardwoods forest; (xxiv) Cades Cove (CC): 35°35'26''N, 83°50'18''W, elevation 535 m, old field near the ATBI plot; (xxv) Tremont (TR): 35°38'30''N, 83°41'48''W, elevation 465 m, hemlock-mixed hardwoods forest.

Five to 35 samples (each 10–30 g) were collected at each site, some of which were visited on more than one occasion. All samples were placed in sterile plastic bags and returned to the laboratories at Shepherd University or Ohio University for processing. Isolation procedures used for dictyostelids were those described by Cavender and Raper (1965a). Each sample was weighed and enough sterile distilled water added for an initial soil/water dilution of 1:10. This mixture was shaken to disperse the material and to suspend the cells of dictyostelids. A 5.0 mL volume of this initial dilution was added to 7.5 mL of sterile, distilled water to create a 1:25 dilution of sample material. For many of the samples pH was determined for this dilution with a Fisher Scientific Accumet AB15 pH meter. Aliquots (each 0.5 mL) of this suspension were added to each of two or three 95–100 x 15 mm culture plates prepared with hay infusion agar (Raper 1984). This produced a final dilution of 0.02 g of soil per plate. Approximately 0.4 mL of a heavy suspension of E. coli was added to each culture plate, and plates were incubated under diffuse light at 20–25 C. Each plate was examined at least once a day for several days after appearance of initial aggregations, and the location of each aggregate clone marked. When necessary isolates were subcultured to aid identification. Nomenclature follows Raper (1984).

The data obtained from the entire set of samples from each study site were used to calculate importance value (IV) indices for each of the species present. They were based on relative density (RD) and relative frequency (RF), where RD = number of clones recorded for a given species/the total number of all clones from the site being considered and RF = total number of samples from which that species was recorded/the total number of samples represented by all species recovered at the site. As used herein, IV for a particular species = one-half the sum of RD and RF.

Data for all sites within each of three elevation zones (1470–1920 m, considered as high elevation; 730–915 m, considered as intermediate elevation; and 465–615 m, considered to be low elevation) were pooled to assess the effects of elevation and the associated changes in vegetation on the general patterns of occurrence of dictyostelids in the park.

	RESULTS

TOP ABSTRACT INTRODUCTION THE STUDY AREA MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
More than 2300 clones of dictyostelids were recovered from the 412 samples collected. These clones included representatives of 20 described species together with at least 10 species new to science (TABLE I). The latter (Acytostelium anastomosans, A. longisorophorum, A. magnisorum, A. serpentarium, A. singulare, Dictyostelium amphisporum, D. potamoides, D. naviculare, D. oculare and D. stellatum) were described in an earlier paper (Cavender et al 2005). Two other species were isolated for the first time in North America. These were D. fasciculatum, which is common in Germany (Cavender et al 1995), and D. firmibasis, which is common in Japan (Cavender and Kawabe 1989). A number of clones could be identified only to genus; in most instances these are likely to have been aberrant examples of one or more of the 30 species referred to above, but the possibility that they represented other species can not be discounted.

Based on pooled data from all sites, D. mucoroides, D. minutum, P. violaceum, P. pallidum and D. discoideum are the most common and widespread species of dictyostelids in the park as a whole. Each had an average importance value (IV) >10 and was recorded from at least half of all study sites. One other species (P. tenuissimum) had a comparable importance value (9.7) but was recorded from just eight sites. Only three other species (D. lacteum, D. aureostipes and D. purpureum) were recorded from as many as 10 sites and nine species were limited to a single site.

Several of the more common and widespread species displayed differences in abundance for the three elevation zones. For example D. discoideum and P. tenuissimum were relatively more common at high elevations, whereas D. aureostipes, D. lacteum, D. purpureum and P. violaceum were relatively more common a low elevations. Because low elevation study sites were characterized by higher values of soil pH than high elevation sites (TABLE I), soil pH conditions probably represent an ecological factor of some importance. The values of pH recorded for soils in the various collecting sites included only one set of samples (from inside Gregory Cave) with a mean pH above 6.5. If more sites had been near neutrality, the pH/species relationship presumably would have been more dramatic. For example in Germany, with an abundance of limestone-derived soils, both Leitner (1987) and Cavender et al (1995) found D. minutum associated with more acidic conditions whereas P. candidum and D. fasciculatum were associated with less acidic conditions. In the generally more acidic soil conditions characteristic of the park, D. minutum, D. mucoroides and P. pallidum were common over a wide range of elevations and soil pH conditions. Thirteen species were limited to or achieved their maximum importance value in sites at high elevations, and the same type of situation applied to eight species for both the intermediate and low elevation sites. Not surprisingly, most of the new species, all of which are represented by limited material, were associated with a single elevation zone. It would seem especially noteworthy that the present study is the first in which a relatively large number of new species, in this case seven (A. longisorophorum, A. serpentarium, D. amphisporum, D. naviculare, D. oculare, D. potamoides and D. stellatum), has been recovered from high elevation sites during a distributional study of dictyostelids. Only a single new species was reported previously from high elevations during comparable studies. Examples include D. polycarpum from the subalpine zone (1600–1800 m) of Switzerland (Traub et al 1981), D capitatum from high elevation (1450–1700 m) coniferous forests in Japan (Hagiwara 1983) and D. crassicaule from Pinus pumila thickets (elevation ca 1950 m) on Mount Chokai in Japan (Hagiwara 1984). Dictyostelium septentrionalis, a species described originally from Alaska (Cavender 1978) and known to have a low temperature optimum for growth and development (Raper 1984), was recorded only from two high elevation sites. It also is known from the subalpine zone on Whiteface Mountain in the Adirondacks (Cavender, unpublished data) and probably occurs on other peaks of the Appalachians.

	DISCUSSION

TOP ABSTRACT INTRODUCTION THE STUDY AREA MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
The dictyostelids of Great Smoky Mountains National Park and other areas within the southern Appalachians of eastern North America have been the subject of several previous studies. While in graduate school at the University of Wisconsin, the third author collected samples for isolation of dictyostelids from six sites in the park (Cavender 1963). Forest types represented by these six sites included oak-pine, mixed oak, hemlock-mixed oak, mixed mesophytic and spruce-fir. Six species of dictyostelids (Dictyostelium discoideum, D. minutum, D. mucoroides, D. lacteum, Polysphondylium pallidum and P. violaceum) were recovered. The data obtained for two of these sites, which included all six of these species, were reported by Cavender and Raper (1965b). At the time this sampling was carried out only about a dozen species of dictyostelids had been described worldwide and some of the forms now considered to represent separate taxa would not have been recognized. As such, the fact that approximately half of all of the species then known to science were collected in the park is much more significant than the actual number of species recorded in this earlier study. Stephenson and Landolt (1987) included one site within the park in their survey of the dictyostelids associated with southern Appalachian spruce-fir forests. This site (on Mount Collins) yielded four species, one of which (D. aureostipes) had not been reported by Cavender. However this species was not described until 1979. Before that date it would have been included within the D. mucoroides complex. Studies similar in scope but carried out elsewhere in the southern Appalachians include that of Cavender (1980), who isolated a total of 14 species (two of which were undescribed) from 19 sites on the opposite, more moist side of the Appalachians, which is generally referred to as the southern embayment. Fourteen of these sites were arranged in order of decreasing elevation 1800–230 m. The optimum environment for species richness was found to be at middle elevations (590–820 m), whereas the highest densities (1050–2140 clones/g) occurred at 590–1450 m. These elevations include the range (660–920 m) over which biotic diversity was highest. In this study there were an average of 6.64 species/site compared to 7.26 species/site in the present study. However some of the species (D. aureostipes, D. implicatum, P. candidum and P. tenuissimum) that we recovered in the park had not been described at the time (in 1975) samples were collected in the earlier study, and had these been included the numbers recorded for species/site probably would have been similar. The same species (D. mucoroides, P. violaceum, P. pallidum, D. minutum, D. discoideum, D. purpureum and D. lacteum) were found to be the most prominent in both studies. Moreover D. minutum and D. discoideum increased in abundance with increasing elevation whereas D. lacteum and D. purpureum were more prominent at low elevations, which was the same pattern observed in the park. Therefore the overall distribution patterns of dictyostelids are probably similar on both sides the Appalachians except that densities in the southern embayment would appear to be much higher (an average of 1005 clones/g/site compared to143 clones/g/site recorded in the present study). This probably is due at least in part to the higher precipitation totals (up to 390 cm/y) characteristic of the southern embayment. However the fact that soil samples in the earlier study were processed (at the Highlands Biological Station) the same day they were collected might be even more of a factor. In the only other published study of which we are aware Landolt and Stephenson (1986) listed six species and two other forms that could not be assigned to any described species from five sites in the vicinity of Mountain Lake in the mountains of southwestern Virginia.

In eastern North America a considerable body of information is available for the states of Ohio and West Virginia, largely as a consequence of the fact that the three authors of this paper have lived and worked in these two states during much of their respective academic careers. Cavender and Hopka (1986) reported 16 species from 85 study sites throughout Ohio, and collecting carried out in the state since then has increased this total by 12, including two new species, Acytostelium magnuphorum (Cavender and Vadell 2000) and Dictyostelium ohioensis (Cavender and Vadell 2006). In addition possibly two other new species have yet to be described (Cavender unpublished data). Landolt and Stephenson (1990) listed 12 species from forest soils at 21 sites throughout West Virginia, with one additional species recovered from samples collected in caves (Landolt et al 1992). Datasets available for other temperate regions of the world include those from India (Cavender and Lakhanpal 1986), Japan (Cavender and Kawabe 1989), two countries in western Europe (Traub et al 1981, Cavender et al 1995) and New Zealand (Cavender et al 2002).

The number of species (at least 30) recorded from the park exceeds the totals reported in any of these other studies. Ohio is characterized by the highest species richness reported to date, with numbers of species for the other regions ranging from 12 (India) to 26 (Japan). Sites from which samples were collected in Japan included some in warm temperate areas of the country, but no samples were collected from regions in southernmost Japan (Kyushu). Based on the results obtained in all these studies, it would appear that the number of dictyostelids one might reasonably expect to recover in any temperate region of the world would be no higher than about 15–20. The figure for Ohio is exceptional in part because the exceedingly high sampling intensity, which certainly exceeds that of any comparable region of the entire world. As such species richness of dictyostelids in the park would appear to exceed that of any other temperate region of the world investigated to date.

Dictyostelids may be divided into three groups on the basis of the size of their sorocarps: large >10 mm, intermediate 3–9 mm and small <2 mm (Cavender et al 2005). All 10 of the new species described from material collected in the park belong to the third category. It has become increasing apparent that small species of dictyostelids are not uncommon in nature, although they tend to be far less conspicuous than the larger and consequently better known species. One might assume that the small species, because of their size and resultant smaller spore numbers, would be at a competitive disadvantage when compared to larger species. Their small size presumably would let them to compete more successfully under conditions that are marginal for larger species. Such would be the case for microhabitats in which the bacterial food supply is limited. Most samples collected for laboratory isolation of dictyostelids traditionally have been obtained from areas of relatively fertile soil in forest communities with little disturbance. However results obtained from recent studies, especially those by the third author in Ohio, have indicated that the microhabitats represented by the margins of bogs, salt marshes and the relatively infertile soils of elfin woodlands, all of which would not appear to be especially favorable for dictyostelids, actually harbor additional species that do not appear to occur elsewhere. In the present study an effort was made to examine such microhabitats, and several of the 10 new species (e.g. Dictyostelium naviculare and D. stellatum) found in the park were associated with a boggy area at the margin of a small high elevation Sphagnum bog at Andrews Bald. Because these same species did not turn up as the result of extensive sampling of other high elevation sites, it appears likely that they are restricted to the microhabitat found only at the margin of the bog. This could result from these species being especially well adapted to the conditions (nutrient poor, acidic soils) that exist there and/or from these species being excluded from more favorable microhabitats by larger species of dictyostelids. To amplify this point it has been noted that seven of the new species were isolated from sites at high elevations. Values of soil pH recorded for these sites ranged were 4.1–5.1. It consequently would be expected that all these sites would be nutrient poor as well as characterized by lower bacterial numbers. A publication by Fierer and Jackson (2006) on the diversity and biogeography of communities of soil bacteria indicates that bacterial diversity is controlled by edaphic variables and that the lowest levels of bacterial richness and diversity are in acidic soils. Experiments carried out by Horn (1971) demonstrated that dictyostelid competition is influenced by the composition of the bacterial food supply and that preference for certain bacteria could account for the coexistence of a number of species in the same microhabitat. At least eight species can occur in the same soil sample in both temperate and tropical forests (Cavender unpublished data). Where this high level of species richness has been found to exist (e.g. The Wilds in Ohio and Tikal in Guatemala), soil pH is close to neutrality or slightly above. The occurrence of the small species of dictyostelids at high elevations in the park probably is not due to an abundance of different kinds of bacteria but rather to their paucity, which might eliminate most of the larger dictyostelids as competitors. This aspect of dictyostelid ecology certainly warrants additional study.

It is possible that certain microhabitats in the park have served as refugia for species that once might have had a wider distribution when climatic conditions were very different in eastern North America. For example Sphagnum bogs are uncommon in the park so the microhabitat represented by bog margins is exceedingly limited. Sphagnum bogs become increasingly more common in more northern regions of eastern North America, and species apparently restricted to the soil conditions that exist at bog margins might not be as rare as the few records from the park would seem to suggest. The isolation of two species (Acytostelium magnuphorum Cavender et Vadell and Dictyostelium quercibrachium Cavender, Vadell, J. C. Landolt et S. L. Stephenson) that are new for North America from the margin of a small bog in Ohio (Cavender and Vadell 2006) supports this conjecture. However, until more such microhabitats are investigated elsewhere, this situation remains problematic. Caves represent one of the more unusual and least studied habitats in which dictyostelids are known to occur (Landolt et al 1992). In the present study only a single cave (Gregory Cave) was investigated. Samples collected in the cave yielded seven species that could be identified along with a number of isolates that could not be assigned to any described species. All the identified species also were recovered from aboveground samples collected elsewhere in the park, but the highest importance value recorded for one species (D. leptosomum) was for the set of samples collected in the cave. In their study of caves in West Virginia Landolt et al (1992) found D. rosarium to be consistently present, although it had never been recovered from any aboveground site in the state. Presumably this is another example in which the distribution of certain species of dictyostelids is restricted to particular microhabitats. Although D. rosarium was not recorded in the present study, the occurrence of this species in other caves within the park would not be unexpected.

In summary the data obtained from extensive sampling carried out throughout Great Smoky Mountains National Park indicates that the biodiversity of dictyostelids is higher than any other temperate region of the world investigated to date. Some of the species present appear to be restricted to unique and spatially limited microhabitats in the park, which suggests that microhabitats that would not seem to be especially appropriate for dictyostelids need to be examined. Although some of the more common and widespread species appear to display distribution patterns that could be related to differences in elevation and/or major forest types, the actual factors that determine the distribution of dictyostelids in nature remain problematic. As such the present study provides a baseline of information that can serve as the basis for future research on dictyostelid ecology.

	ACKNOWLEDGMENTS

 
This study was supported by grants from the Discover Life in America Foundation, the Shepherd University Foundation and Alumni Association, the West Virginia NASA Space Grant Consortium and the National Science Foundation (Grant DEB-0316284). Ian Stocks, Chuck Parker, Randy Darrah, Paul Davison, Will Reeves, Melinda Landolt and Jeanie Hilten contributed sampling assistance to this study and Nancy Critzer assisted in laboratory isolation procedures.

	FOOTNOTES

 
Accepted for publication May 22, 2006.

¹ Corresponding author. E-mail: jlandolt{at}shepherd.edu

	LITERATURE CITED

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