Student Papers
Raven and Johnson's Biology, Sixth Edition

Growth in Earthworms

Student Research Project
Effects of soil temperature and moisture on multilocus heterozygosity-growth relationship in the earthworm Eisenia fetida

Students
Melinda Chow
Major: Biology
Future Plans: Graduate school

Tara Mann
Major: Biochemistry and Molecular Biology
Future Plans: Graduate school

Yonya Nabors
Major: Microbiology
Future Plans: Graduate school

Theresa Polk
Major: Biological Engineering
Future Plans: Biomedical implant design industry

Professor
Walter J. Diehl, Associate Professor, Department of Biological Sciences, Mississippi State University, Mississippi State

Multilocus heterozygosity is the cumulative number of heterozygous loci in a set of polymorphic loci in an individual. Using the lumbricid earthworm Eisenia fetida as a model organism, we are testing the hypothesis that growth is correlated with multilocus heterozygosity in stressful environments but not in nonstressful environments. This research, funded by the National Science Foundation, will help explain how genotype by environment interactions contribute to individual phenotypes in an important group of terrestrial organisms. It also has the potential to contribute further to understanding the process of stabilizing selection that preserves genetic variation in populations.

Four undergraduate students, Melinda Chow, Tara Mann, Yonya Nabors, and Theresa Polk, two graduate students, and I are examining the effects of soil temperature and moisture on relationships between multilocus heterozygosity and growth in E. fetida. Because soil temperature and moisture are two of the most important environmental variables affecting growth and distribution of earthworms, they are appropriate factors to manipulate in the laboratory. Earthworms were raised individually in combinations of four temperatures (15, 20, 25, 28° C) and three soil moistures (2, 3, 4 ml H2O/g dry neutral peat moss). The fresh weight of each earthworm was measured every two weeks until six weeks after hatching. Environmental stress was indicated by the depression in growth rate in a treatment compared to that in the control treatment (25° C, 4 ml H2O/g). Starch gel electrophoresis was used to determine multilocus heterozygosity of each earthworm at seven polymorphic loci encoding enzymes that catalyze reactions in glycolysis, Krebs cycle, pentose shunt, lipid metabolism, and protein metabolism.

Maximal growth rates (measured as change in fresh weight) occurred in the control treatment (25° C, 4 ml H2O/g). Growth rates declined as temperature increased (28° C) or decreased (15, 20° C) or as moisture decreased (2, 3 ml H2O/g). To date, we have observed correlations between individual multilocus heterozygosity and growth in earthworms from some of the treatments that depressed growth. Multilocus heterozygosity tended to be correlated with growth at temperatures near the upper thermal limit for the species (25, 28° C) but not at low temperatures (15, 20° C). At the upper temperatures, multilocus heterozygosity tended to be correlated with growth at moderate soil moistures (3 ml H2O/g). These results suggest that multilocus heterozygosity-growth relationships are not related to the degree of environmental stress per se but rather depend on both the quality and quantity of environmental stress. The data reinforce the fact that different environmental factors usually affect bioenergetic processes by different mechanisms even if extreme levels of each factor can depress growth rates to the same extent. In the future, we will examine correlations between multilocus heterozygoisty and fecundity/survivorship in the species.

Return to Main Page