Macroevolutionary patterns of sexual size dimorphism in copepods

Area of Science:

• Evolutionary Biology
• Zoology
• Animal Behavior

Background:

• Sexual size dimorphism (SSD) is a widespread phenomenon in animals, with major evolutionary theories attempting to explain its macroevolutionary trends.
• Quantitative genetic theory and Rensch's rule (RR) propose specific patterns of size variation and covariation related to directional selection and sex-specific allometry.
• Copepods, exhibiting diverse SSD patterns, provide a valuable model system for testing these competing hypotheses.

Purpose of the Study:

• To test competing macroevolutionary theories explaining sexual size dimorphism (SSD) in animals, specifically in copepods.
• To investigate the relationship between allometric slopes, SSD, and the predictions of quantitative genetic theory and Rensch's rule.
• To examine the influence of adult sex ratio on SSD variation in free-living and parasitic copepods.

Main Methods:

• Comparative analysis of sexual size dimorphism (SSD) across diverse clades of free-living pelagic and parasitic copepods.
• Examination of allometric slopes (male to female size) and their covariation with SSD.
• Statistical testing of the hypothesis that adult sex ratio influences SSD, comparing free-living copepods with parasitic species like sea lice (Caligidae).

Main Results:

• Widespread isometry was observed, providing no support for Rensch's rule or the predicted covariation between allometry and SSD.
• Selection on both sexes appears to be equally important in shaping SSD, contrary to theories emphasizing stronger selection on one sex.
• No correlation was found between adult sex ratio and SSD variation in free-living copepods, challenging the hypothesis that reduced male abundance lowers male size.
• Parasitic copepods, particularly sea lice, showed some support for the sex ratio hypothesis, but other parasitic groups with similar SSD suggest alternative drivers for extreme dimorphism.

Conclusions:

• Existing theories of sexual size dimorphism (SSD), including Rensch's rule and predictions from quantitative genetics, are not universally supported by patterns observed in copepods.
• Equal selection pressures on both sexes likely play a significant role in shaping SSD.
• The influence of adult sex ratio on SSD is complex and may be context-dependent, with other factors driving extreme dimorphism in certain parasitic species.