Aims of the Paper
The aim of the study was to investigate the genetic process behind craniofacial adaptation and diversity in cichlid fish to understand the specific DNA mutations, genetic mechanisms, and molecular changes contributing to remarkable variation in the craniofacial morphology in this species. This study was inspired by the astounding diversity in facial structures among vertebrates of the same species. While it is known that mutations in gene expression are linked to natural changes in facial structure and that neural crest cells derive craniofacial skeleton, the specific changes in the DNA that result in facial morphology evolution remain unknown, creating a gap in research. Powder et al. (2014) sought to close this gap in research. The Cichlid fish are known for their adaptive radiation since they diversify and adapt to various ecological niches rapidly, creating a wide range of morphological variation in jaws and faces. This factor made them the ideal subject of an experiment to understand the DNA mutations driving species diversification.
The Experimental Design, Results, and the Authors’ Conclusions
The researchers hypothesized that the limb bud and heart homolog (lbh), a nonsynonymous mutation in transcriptional coactivator that is linked to adaptive variation in the lower limb of the Cichlid fish, play a role in craniofacial development, thereby contributing to the diversity of these fish. To test this hypothesis, the researchers used a zebrafish and Xenopus collected from the rift valley water bodies and samples extracted for genetic analysis. Genotyping was done to determine the genetic variation in the samples. Powder et al. (2014) then conducted a phenotypic analysis of the craniofacial structures for the samples with special reference to the jaw length, part of the geometric morphometric that quantifies the craniofacial difference to enable comparison. Based on existing literature, the researchers knew that the divided coding allele is only be found in Labeotropheus fuelleborni (LF). Since LF influences one side of the continuous craniofacial variation among the species, they could hypothesize that lbh and R>Q coding mutation, in particular, contributed to the observed adaptive mandible morphology.
A mutation analysis was conducted on the lbh gene in the collected fish sample to determine the mutations present in individuals and populations. Functional studies were conducted to test the hypothesis: First, in vivo, Powder et al. (2014) depleted lbh in zebrafish using translation and splice-blocking morpholinos. Second, MO knockdowns in zebrafish NCC reporter lines were conducted, and defects were observed in cranial development to determine the specific cellular mechanism that leads to NCC development regulation by Lbh. Other manipulations were conducted along with gene expression analysis. Statistical analysis of the data was then conducted.
The researchers found that there was an amino acid change in Lbh that was significantly associated with continuous, adaptive morphological variation. Embryonic patterns correlated with adult phenotypes, and the differences in mandible length were traced back to even before chondrogenesis, in the early stages of craniofacial development. The data from functional analysis, head shape analysis, and jaw lengths showed that amino acid polymorphism affects patterns of NCC migration. From these observations, Powder et al. (2014) concluded that the single amino acid change that was alternatively fixed in Cichlids with different facial morphologies resulted in discrete shifts in the multipotent cell type migration patterns consistent with phenotypes in embryos and adults. Further, cellular events sic as apoptosis, proliferation, and differentiation lead to species-specific differences in facial shapes consistent with the development theory where the phenotype is determined by earlier events. Finally, since the coding mutation occurs in a disordered domain and the derived allele leads to a loss of the predicted short linear motif, Powder et al. (2014) conclude that this supports the theory that modules within coding regions, including those mediating interactions between proteins, show evolutionary plasticity by a high magnitude.
Conclusion and Ideas about Future Experiments
The research results support Powder et al. (2014)’ conclusions because of the significant relationship between changes in the amino acid and continuous, adaptive morphological variation. The methodology is sound, ruling out other reasons for the observations through tests. The differences in NCC migration as the variables are changed support Powder et al. (2014)’ hypothesis. Future studies should focus on using animal models to determine the molecular mechanism through which lbh mutation affects craniofacial morphology. In addition, other species could be used in a similar methodology to determine other genes that are influential in craniofacial diversity.
Reference
Powder, K. E., Cousin, H., McLinden, G. P., & Albertson, R. C. (2014). A Nonsynonymous Mutation in the Transcriptional Regulator lbh Is Associated with Cichlid Craniofacial Adaptation and Neural Crest Cell Development. Molecular Biology and Evolution, 31(12), 3113–3124. https://doi.org/10.1093/molbev/msu267