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Abstract:Side orientation of cells is usually ambiguous in unicellular organisms, making it impossible to separate components of directional asymmetry (DA) and fluctuating asymmetry (FA). However, frustules of the diatom Luticola poulickovae have biradially symmetric outlines, and their central areas bear ornamentation that is asymmetric across the apical axis. The goal of this study was to explore differentiation of morphometric asymmetry across the apical axis into DA and FA components. Is there detectable DA of the valve outlines of two L. poulickovae strains that may be related to the asymmetric central areas? Given that the life cycle of diatoms involves cell-size diminution, and cell shape is strongly affected by allometry, we also explored the question of whether asymmetry is correlated with cell size. The extent of symmetric variation among individuals in each strain, as well as DA and FA across the apical axis, were quantified using two Procrustes ANOVA models. The results revealed no correlation of either total asymmetry or FA with valve size. DA was significant and considerably more pronounced than FA in both strains, indicating that there is previously unknown systematic asymmetry of valve outlines of L. poulickovae, which may be related to the asymmetry of its central area.

The tolerance to pentachloronitrobenzene (PCNB) of an isolate of Sclerotium rolfsii collected in 1985 was quantified, and a survey of tolerance to PCNB in 377 other isolates of the fungus collected from Texas peanut fields from 1990 through 1994 was conducted. The effective dose (ED)50 of the previously collected PCNB-tolerant isolate was 11.07 μg PCNB/ml and was more than 5-fold greater than the ED50 of PCNB-sensitive isolates. The distribution of tolerance to PCNB among all isolates was slightly skewed, with 18 of the 377 isolates identified as having greater (P ≤ 0.05) tolerance to PCNB than the standard sensitive isolate. No isolate of S. rolfsii collected during the period of 1990 to 1994 had as high an ED50 value as did the 1985 isolate, even among those isolates collected from the same field from which the 1985 isolate was collected. ED50 values of two PCNB-sensitive and five PCNB-tolerant isolates were unchanged after 15 generations on potato dextrose agar amended with 10 μg PCNB/ml or on unamended media. The PCNB-tolerant isolate collected in 1985 was less aggressive than other isolates in greenhouse and microplot tests, but no correlation was observed between ED50 values and disease incidence in these tests for other PCNB-sensitive and tolerant isolates. These data suggest that even though high levels of tolerance to PCNB can be confirmed in some isolates of S. rolfsii, this phenomenon is likely to remain a rare event.