After watching the video above, how do you think cichlids vary in mouth sizes? Now this blog entry may not explain the variation in that particular species of cichlids, or tell you anything about cichlid mouth fighting. But this entry will try to explain craniofacial differences in other cichlids.
There are more than a 1000 different cichlid species which makes cichlids a good model organism in evolutionary biology. The study of craniofacial diversification in cichlids is important because it gives us an opportunity to study the evolution of traits. The large scale of cichlids allows us to study microevolution within the species and macroevolution among the genera.
It was discovered that the alleles of the hedgehog pathway receptor Patched1 (Ptch1) gene causes adaptive radiation in Lake Malawi cichlid fish (Roberts 2011).
- Also known as Ptch1
- Provides instruction for producing Ptch1 proteins (receptors)
- Sonic hedgehog is a ligand for Ptch1 receptor
- The function of the receptor and ligand is to trigger signals in cell development
Adaptive radiation is when a species evolve and adapt to survive different environments, which was famously studied by Darwin. Macroevolution is the evolution between gene pools. And microevolution is evolution within the species. By understanding genes that affect craniofacial structures, we can slowly understand the evolutionary journey of other vertebrates.
Since there is an abundant diversity of cichlids, many of them have different ways of feeding and the type of craniofacial structure defines how they feed. This paper looks at the genera Labeotropheus and Metriaclima. Their mouth and jaw shape differ significantly, and this difference in the shape of the mouth shows the macroevolutionary relationship between the species.
Through a comparative genome scan of the two genera, they found that the Single Nucleotide Polymorphism (SNPs) Aln100281_1741 had a high differentiation (Fst) of 0.86. A high Fst may mean that they may share genetic loci with evolutionary divergence between the two genera (Roberts 2011). Through a BLAST search, they were able to find Aln100281 and Ptch1 in another cichlid (Oreochromis niloticus). Through comparative tests they placed the sequence scaffold on the linkage group (LG) 12 on the cichlid genetic map.
Quantitative trait loci (QTL) maps out stretches of DNA and locates sequences that are linked to a quantitative trait. The authors genetically mapped the two cichlid fishes and found that variance in the Ptch1 gene affects the retroarticular process (RA) which in turn affects the oral mechanical advantage (MAO).
The LG12 QTL did not contribute to the outlever length, but contributed to the shortening of the RA which increases the mechanical advantage of the jaw.
Difference in Ptch1 Expression
The authors did not find any different polymorphisms in the Ptch1 coding sequence between the two genera. They did find that Ptch1 expression correlated with suction and biting morphologies. In the Labeotropheus fuelleborni (LF), they found large amounts of Ptch1 expression in the areas where RA will form. The Metriaclima zebra (MZ), which generally have a shorter RA, had much lower levels of Ptch1. However in terms of tail development, both genera had relatively similar levels of Ptch1.
Hedgehog Pathway Manipulation
To further test their results, they manipulated the hedgehog to see how jaw development will change. The authors treated LF larvae with cyclopamine during stage 12 of larval development. Cyclopamine or ethanol (control) was added to the larval fish water.
- Hedgehog signalling pathway inhibitor
- Useful in the study of cancers in which the pathway is overexpressed
Stage 12 is important for craniofacial development and Ptch1 expression. The cyclopamine treated larvae dramatically decreased the expression of Ptch1 and its downstream target Gli1. Normally Ptch1 and Gil1 are in areas with bone differentiation marker Col1a1. The authors also found that there was less Col1a1 expression around the RA. The treatment of cyclopamine significantly reduced the length of the RA, but it did not affect the length of the lower jaw. The authors believe that there are two alleles of Ptch1 that affect development. The long allele increases expression of Ptch1 which increases the length of the RA and MAO, while the short allele will do the reverse. They find that there is no significant relationship between Ptch1 and the length of thickness of the jaw, which means that Ptch1 only affects the RA length.
If you would like to see a more extensive view on levels of Ptch1 expression and the effects of cyclopamine please see the Supporting Information from the paper.
Microevolution and Microevolution
The authors also looked at a third genus of cichlids (Tropheops). The tropheops are an interesting genus because they share characteristics of both Labeotropheus and Metriaclima. Tropheop species in deep water locations have the short Ptch1 allele which makes them adapted for suction feeding. The tropheops in shallow water carry different morphologies. The shallow water tropheops with the long allele has jaw morphology similar to Labeotropheus. And tropheops, who are homozygous for the Ptch1 short allele, has morphology similar to suction feeders. Based from this data the authors were able to show that Ptch1 alleles are responsible to macroevolutionary changes between genera and microevolutionary changes in species.
The paper did a good job telling us the importance of Ptch1 in the craniofacial development. However it did not mechanistically tell us how it affected development. The paper supports the how Ptch1 affects craniofacial development with results, but molecular proof would be more convincing. The pictures did a good job visually showing the different levels of expression between the genera. The paper did not do a great job of explaining QTL and how to read it. It would also be nice to learn how Ptch1 affects the development of other organisms such as humans.
Roberts, R. B., Hu, Y., Albertson, R. C., & Kocher, T. D. (2011). “Craniofacial divergence and ongoing adaptation via the hedgehog pathway.” Proceedings of the National Academy of Sciences of the United States of America, 108(32), 13194–13199.