DEVELOPMENTAL CONSEQUENCES OF AN EVOLUTIONARY CHANGE IN EGG SIZE: AN EXPERIMENTAL TEST

Area of Science:

• Marine Biology
• Developmental Biology
• Evolutionary Biology

Background:

• Two sea urchin species, *Strongylocentrotus droebachiensis* and *S. purpuratus*, exhibit distinct larval forms and developmental rates.
• A significant difference in egg size exists between these species, prompting investigation into its role in developmental divergence.

Purpose of the Study:

• To experimentally determine if interspecific differences in sea urchin larval form and development rate are caused by differences in egg size.
• To investigate the causal relationship between egg size (parental investment) and early life-history traits in marine invertebrates.

Main Methods:

• Experimental reduction of egg size in *S. droebachiensis* by isolating blastomeres from embryos.
• Quantification of the developmental rate of feeding larvae using a novel morphometric method.

Main Results:

• Smaller eggs resulted in larvae with smaller body size and simpler forms.
• Reduced egg size led to slower development rates through early feeding larval stages.
• The influence of egg size on larval traits was confined to early developmental stages, with experimentally reduced eggs of the larger species exhibiting developmental rates similar to the smaller species.

Conclusions:

• Cytoplasmic volume of the egg, reflecting parental investment, is the primary determinant of early larval form and developmental rate, rather than species-specific genetic differences.
• Epigenetic effects of egg size on larval morphology have significant functional consequences for feeding capability.
• Adaptive evolution of egg size and larval form are interconnected, suggesting that changes in one can drive concerted changes in the other, potentially constraining evolutionary pathways.