Lifetime inbreeding depression, purging, and mating system evolution in a simultaneous hermaphrodite tapeworm

Area of Science:

• Evolutionary biology
• Genetics
• Parasitology

Background:

• Mating system evolution in simultaneous hermaphrodites is typically predicted to favor either outcrossing or self-fertilization based on inbreeding depression (ID) levels.
• However, many species exhibit a mixed mating system, with tapeworms like Schistocephalus solidus self-fertilizing some eggs despite experiencing ID.
• The evolutionary success of selfing depends on the fitness of selfed offspring and the capacity to purge genetic load.

Purpose of the Study:

• To investigate the fitness consequences of two consecutive generations of selfing in Schistocephalus solidus.
• To determine the extent of inbreeding depression and the potential for purging deleterious alleles in this parasitic tapeworm.

Main Methods:

• Breeding Schistocephalus solidus through two successive generations of self-fertilization.
• Measuring various fitness correlates across the entire life cycle for both selfed and outcrossed progeny.
• Quantifying inbreeding depression and estimating the load of lethal equivalents.

Main Results:

• One generation of selfing resulted in pronounced inbreeding depression, with selfed progeny exhibiting only 9% of the lifetime fitness of outcrossed controls, particularly affecting early-life traits.
• A second generation of selfing showed a significant reduction in ID across several traits, with a 48% decrease in the estimated load of lethal equivalents, indicating purging of deleterious recessive alleles.
• Despite purging, inbreeding depression remained substantial, suggesting it could impede a complete transition to self-fertilization.

Conclusions:

• Severe inbreeding depression, even with purging, likely prevents obligate selfing in Schistocephalus solidus.
• While purging of deleterious alleles occurs, the persistent high ID suggests limitations on the spread of self-fertilization.
• The possibility of undetected benefits of low-level selfing cannot be entirely ruled out.