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Genotypes of Multiple Allozyme Loci Interact With an Experimental Environment to Affect Growth in Juvenile Earthworms (Eisenia fetida andrei)

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Abstract

Biweekly growth and genotypes at seven polymorphic allozyme loci were measured for juvenile Eisenia fetida andrei raised in all combinations of three soil moistures and four temperatures for 10 weeks (n = 1062). There was a significant interaction (P < 0.05) between genotype and the environment that affected growth (repeated measures ANOVA) at six of these loci after the effect of environment alone was taken into account. Genotype × environment interactions for each locus were evaluated by computing the relative growth of each genotype (mean growth of the genotype divided by the maximal mean growth among genotypes) within each environment. For three of six loci, the mean relative growth of the heterozygous genotype among environments was greater than that of the homozygous genotypes. The difference in relative growth among genotypes and environments may be sufficient to maintain polymorphisms of some allozyme loci in earthworms that experience environmental heterogeneity if each combination of environments is inhabited equally and if relative growth is a good predictor of fitness.

Introduction

The physiological consequences of variation at single allozyme loci have been examined in detail in remarkably few species [reviewed by 22, 38]. Notable examples that demonstrate cause and effect relationships include studies of the alcohol dehydrogenase locus in Drosophila melanogaster 23, 24, 26, the lactate dehydrogenase locus in Fundulus heteroclitus 5, 10and the aminopeptidase I locus in Mytilus edulis 18, 19. These cases show genotype × environment interactions where selection maintains polymorphisms of each locus in natural populations.

Examining the physiological consequences of variation at multiple allozyme loci has been restricted to studies of multilocus heterozygosity (MLH)-fitness/growth relationships in various species. [Most recently reviewed by 27, 28]. Koehn et al. [20]postulated that the environment plays an important role in promoting these associations, a hypothesis that has been tested in only a few cases. The environment may provide a necessary degree of general stress to maximize the energetic benefits of heterozygosity [2]. MLH-fitness relationships would occur if loci typically show overdominance in moderately stressful environments regardless of the specific environmental factors that cause stress. Alternatively, the environment may provide heterogeneity of specific variables relevant to the metabolism of the species 22, 32. MLH-fitness relationships would occur if a sufficient number of loci show overdominance (or even marginal overdominance) in a given pattern of environmental heterogeneity. If so, MLH-fitness relationships could be both locus and environment specific even within species.

Any relationship between MLH and fitness must be a special case of a more general relationship between genotypic variation at multiple loci and fitness. Detecting MLH-fitness relationships succeeds because genotypic variation is reduced to a single index (MLH) based on heterosis theory, and thus analyses make maximal use of available samples. Other patterns of genotypic effect cannot be reduced to so convenient an index and require much larger samples and multiple environments for detection (e.g., adaptive distance models) [3].

Attempts to detect genotype × environment interactions by experimental perturbation of the environment have concentrated on analyses of quantitative trait loci (QTLs) [reviewed by [16]]. These studies have shown that interactions between genotypes and the environment affect the same fitness characteristics measured in MLH-fitness studies. Identifying specific genes corresponding to QTLs has met with little success, even though narrow regions of chromosomes containing such loci have been identified [e.g., [34]], but see [36]. Either way, QTLs do not usually map to the same locations on chromosomes as allozyme loci [e.g., [35]].

Little effort has been directed toward examining genotypic effects at multiple allozyme loci on fitness. A notable exception is the study of Vrijenhoek et al. [37], who showed that survivorship of the topminnow Poeciliopsis monacha was a function of both allozyme genotype and environmental stress. Because of the lack of studies in this area, questions remain concerning whether variation at allozyme loci affects individual fitness generally and to what extent this may contribute to heterosis.

We report the results of an experiment to examine the effects of multilocus genotypic variation (as opposed to multilocus heterozygosity) on growth of juvenile earthworms (Eisenia fetida andrei) raised in combinations of constant soil moisture and temperature. We tested the hypothesis that variation in genotypes of multiple allozyme loci would interact with the environment to affect growth. This experimental approach randomizes historical events, drift and effects of background loci among environmental treatments and eliminates effects of gene flow on any relationship between multilocus genotypic variation and fitness. Relationships among MLH, environmental heterogeneity and growth is considered elsewhere.

Inbreeding has the potential to produce patterns that mimic some types of genotype × environment interactions in allozyme loci. E. f. andrei shows little evidence of inbreeding. Earthworms were sampled from a stock population containing 5000–10,000 earthworms (1350–2700/m2), depending on the season. Selfing does not produce viable cocoons [30], despite a report to the contrary [15]. Attempts to create inbred lines by raising pairs of siblings in isolation produced offspring at a very low rate, and none of these survived to maturity (Diehl, personal observation). Lack of inbreeding is supported by genetic data 6, 8, including high polymorphism (P = 0.33), high average heterozygosity (Ho = 0.40) and low average heterozygote deficiency (D = −0.066). The observed distribution of heterozygosity classes was not significantly different from the expected distribution [6]. These results are also inconsistent with aneuploidy, molecular imprinting or null alleles in the population 6, 12. Thus, no other genetic phenomena is a likely cause of any genotype × environment interactions detected.

Section snippets

Materials and methods

E. f. andrei were raised in all combinations of three soil moistures (2, 3, 4 ml H2O/g dry peat moss) and four soil temperatures (15, 20, 25, 28°C). Treatments were initiated with 200 newly hatched earthworms weighing about 20 mg each (about 3 weeks old), except for the 25°C treatments that were initiated with 100 earthworms. Dead earthworms were replaced with other newly hatched earthworms in some treatments to maintain sample sizes for other experiments (total initial sample size, 2300).

All

Results

The effects of an interaction between genotype at seven allozyme loci and the environment on juvenile growth in E. f. andrei are reported in Table 1. There was a significant genotype by environment interaction (after effects of environment alone were taken into account) for six of seven loci examined. Only the Aap locus showed no effect on juvenile growth. One cannot determine from this analysis whether the significant effects were heterotic, similar among environments or similar among loci.

Discussion

Genotypes of six of seven allozyme loci interacted with an experimentally controlled environment to affect growth in E. f. andrei. The interaction term permits genotypic responses to vary differently among habitats such as along environmental gradients. Consequently, a significant genotype × environment interaction is a more sensitive indication of genotypic effect than a main effect of genotype alone. Of the loci tested, only Aap showed no genotype × environment interaction. For loci showing

Acknowledgements

We thank Rebecca Brasfeild, Melinda Chow, Warren Cox, April Heinsch, Carie Loundenslaqer, Tara Mann, Yonya Nabors and Theresa Polk for assistance in data collection and earthworm husbandry. This research was supported by NSF grant DEB-9221094 to W.J.D.

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