Internal selection against the evolution of left-right reversal

Area of Science:

• Developmental Biology
• Evolutionary Biology
• Genetics

Background:

• Left-right asymmetry reversals are rare in metazoans, implying mechanisms suppress their fixation.
• External symmetry and midline reproductive organs suggest minimal disadvantages for reversed individuals in the wild.
• Endogenous fitness reduction is hypothesized to prevent the widespread occurrence of reversed species.

Purpose of the Study:

• To investigate the endogenous mechanisms suppressing left-right polarity reversal in metazoans.
• To understand how selection operates against reversed asymmetry.
• To analyze the phenotypic consequences and genetic basis of reversed asymmetry.

Main Methods:

• Generated dextral and sinistral snails with identical zygotic genotypes by exploiting hermaphroditism and maternal inheritance of polarity.
• Compared developmental anomalies and shell morphology between sinistral and dextral snails before hatching and at maturity.
• Analyzed interchiral differences in shell shape and variation between parental crosses.

Main Results:

• Sinistral snails exhibited significantly higher rates of lethal morphological anomalies before hatching compared to dextral snails.
• Mature sinistral snail shells were not mirror images of dextral shells, indicating phenotypic changes beyond simple reversal.
• Interchiral differences varied between parental crosses, suggesting epistatic effects on sinistral snail performance.

Conclusions:

• Internal selection actively operates against left-right polarity reversal in snails.
• The observed pleiotropy in maternal effects or linked genes contributes to the disadvantage of reversed snails.
• Changes in blastomere configuration are proposed as a potential cause for endogenous selection against reversed polarity.