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and VRIJENHOEK 1986). For the triploid parthenogenetic form of the woodlouse Trichoniscus pusillus, CHRISTENSEN. (1979) demonstrated the existence of at ...
Hereditas 108: 213-21 7 (1988)

Differential responses to humidity and soil type among clones of triploid parthenogenetic TrichoniscuspusiZZus (Isopoda, Crustacea) BENT CHRISTENSEN, HENNING NOER and BENT FRIIS THEISEN Institute of Population Biology, University of Copenhagen, Denmark

B., NOER, H. and THEISEN, B. F. 1988. Differential responses to humidity and soil type among clones of triploid parthenogenetic Trichonrscuspusiilus (Isopoda, Crustacea). -Hereditas 108: 213217. Lund, Sweden. ISSN 0018-0661. Received July 3,1987

CHRISTENSEN,

In an attempt to clarify the background of differential distributions of Trichoniscus pusillus clones in the field, two series of laboratory preference experiments were undertaken. One tested clonal reactions towards respectively increasing and decreasing moisture content of substrate, the other, distributions of clones over pairs of selected types of soilllitter. The results of the moisture preference experiments showed highly significant differences in behaviour towards extreme water conditions. These results indicate that moisture is an important ecological factor underlying clonal distributions. The observed patterns in behaviour are in accordance with observed temporal fluctuations in clonal frequencies in nature, but cannot explain that in respectively dry and wet habitats clones with different types of behaviour tend to coexist. The soil type preference experiments showed significantly different responses among clones. In general, however, some clones responded while others did not, and “true” differential responses were not found. Even then, these results indicate that differential microhabitat preference is of potential importance for the coexistence of clones. Bent Christensen, Institute of Population Biology, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen 0 , Denmark

Recent studies have provided much evidence that ers-in clonal composition of asexual species the genetic diversity of parthenogenetic species gen- (CHRISTENSEN 1979 and 1980; WEiDER 1984; SCHENCK 1986). erally is adaptive, and that the clonal composition of and VRIJENHOEK For the triploid parthenogenetic form of the natural populations is maintained by selection (SUOMALAINEN et al. 1976; CHRISTENSEN 1979 and woodlouse Trichoniscus pusillus, CHRISTENSEN 1980; MITTERet al. 1979; VRIJENHOEK 1979and 1984; (1979) demonstrated the existence of at least four HEBERT and CREASE 1980; and LYNCH 1983). How- clones (termed I-IV). For 5 pairs of stations, in ever, in virtually all investigated cases it is less well beech and spruce habitats respectively, clonal comunderstood how this selection operates in details, positions differed significantly even though each i.e., exactly what components of fitness and what pair was spaced by only 5-10 m. This situation could genotype-environment interactions are involved in be maintained if the clonal variation is adaptive, but if the animals disperse freely, selection would have the coexistence of clones. Besides the traditional Darwinian components of to be very strong to conserve such sharp gradients. selection, it is well known that differential habitat Evidence that the clonal variation in T . pusillus is preference of genotypes facilitates the conservation adaptive is presently accumulating. Clones show of genetic variation in a heterogeneous environ- constant habitat associations over time and space ment. For sexual outbreeders it may be difficult to and, in particular, 11 stations investigated annually maintain the constant combination of traits re- 1980-1984 showed highly significant simultaneous quired by this strategy because of recombination year-to-year changes in relative frequencies of (e.g., HEDRICK 1986), but in organisms with clonal Clones (CHRISTENSEN 1983; CHRISTENSEN and NOER propagation this obstacle is probably no greater 1986). CHRISTENSEN and NOER(1986) suggested that problem. As a consequence, differential habitat these temporal fluctuations are due to the influence preference must be considered a factor of potential of drought periods, but this hypothesis rests on corimportance in the coexistence of clones. This seems relations and is thus in need of support by direct exto be borne out by a number of reported cases of perimental evidence. CHRISTENSEN et al. (1987) microspatial variation-an scales down to 2-4 met- found highly significant differences in brood sizes

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Hereditas 108 (198X)

between clones, but these differences were sup- gradually increased by adding small amounts of posed to be too small to counterbalance the effects water at regular intervals. Individual reactions were of a random dispersal. Similarly, differences in the scored as described above, but in general these exsurvival of clones would have to be unrealistically periments provided less clear-cut responses since a strong in order to maintain the observed balance. fair proportion of the animals tended to sit tight Obviously, the existence of differential habitat pre- when experiencing an increasing wetness. These ferences between clones would make their distribu- were scored as “not responding” at the termination of the experiments-generally after two hours. tions and coexistence much more understandable. The soil type preference experiments were carThe purpose of this paper is to present the results of two series of experiments exploring into this pos- ried out in an arena measuring 11.5x 94 cm. The two sibility. The first of these series was designed to in- halves of this arena were covered with 1 cm layers of vestigate whether clones act differentially towards top soil and fallen leaves from different habitat gradients in moisture conditions, while the second types. At the start of each experiment, several and more preliminary series was an attempt to in- hundred T. pusillus were released evenly over the vestigate whether differential habitat preferences area. Following release they were allowed to wanexist with respect to other factors. Results for a der freely at a temperature of 10-12°C for four days. During this period, the arena was covered with a coexisting diploid form of the species (CHRISTENSEN 1983) are also presented and used throughout as a tight lid in order to prevent desiccation and to secure complete darkness. control. In all types of experiments, indiviquals were assigned to clones by means of their electrophoretic 1979 and profiles at the Pgm locus (CHRISTENSEN Methods 1983). Clonal reactions to water stress were investigated by measuring the sequence in which individuals left environments with unfavourable water conditions. The two halves of a circular filter paper were placed Results 1 mm apart in a 10cm Petri dish. One half was made wet, the other very wet, and 25 individuals of T. Results of reactions towards decreasing moisture pusillus were introduced. During an initial period content are presented in Fig. 1. Overall compariwhich lasted up to 1 hour these individuals moved sons of diploids and the four investigated clones reabout, but eventually they all settled in the wet half, vealed highly significant differences (x2= 119.7, clearly avoiding the very wet. When reactions to- df=20, P