Gene network variation and alternative paths to convergent evolution in turtles.

Area of Science:

• Evolutionary developmental biology
• Comparative genomics
• Vertebrate paleontology

Background:

• Turtle shell diversification involves significant phenotypic changes.
• Convergent evolution of shell-closing systems in divergent species highlights unique scapula segmentation.
• Skeletal segment formation is an evolutionarily conserved developmental process, suggesting conserved genetic underpinnings.

Purpose of the Study:

• To investigate the molecular mechanisms underlying convergent evolution of turtle scapula segmentation.
• To compare gene co-expression networks associated with scapula development across different emydid turtle species.
• To test the hypothesis that similar phenotypic changes arise from similar genetic network alterations.

Main Methods:

• Comparative transcriptome analysis of scapula tissue from three embryonic developmental stages in three emydid turtle species.
• Gene co-expression network analysis to identify key regulatory genes and pathways.
• Differential gene expression analysis against an ancestral lineage reference.

Main Results:

• Interspecific differences in gene co-expression networks were associated with alternative skeletal segmentation strategies.
• Mesenchyme homeobox 2 (MEOX2) and HOXA3-5 were identified as central hubs in one species' scapula segmentation network.
• Similar myogenic networks were active in species with scapula muscle overgrowth, despite differences in segmentation mechanisms.
• The ancestral, unsegmented scapula condition in the third species lacked these derived developmental processes.

Conclusions:

• Convergent evolution of complex traits like the turtle shell is influenced by variations in underlying developmental processes and gene networks.
• While some molecular pathways may differ, conserved developmental processes like myogenesis can contribute to convergent phenotypes.
• This study provides insights into the molecular basis of evolutionary novelty and the diversification of the unique turtle body plan.