Females of many invertebrates contain stored sperm or fertilized eggs or both, causing potential genotyping errors. We investigated errors caused by male DNA contamination by amplifying 5 microsatellites in DNA isolated from various tissue types in the nematode Ascaris lumbricoides. We observed additional alleles in 30/135 uterus-derived samples when compared with muscle controls, resulting in 20/135 (15%) incorrect genotypes and an underestimation of inbreeding. In contrast, we observed additional alleles in only 5/143 ovary-derived samples, resulting in 4/143 (3%) incorrect genotypes and no significant influence on inbreeding estimates. Because uterus constitutes ∼17% of a female's organ weight, a substantial proportion of samples isolated from female tissue may contain male-derived DNA. Male contamination is easily avoided when using large nematodes such as A. lumbricoides. However, we urge caution for studies using DNA isolated from small invertebrates that store sperm or fertilized eggs or both.

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