From the lab of George Turner (Bangor University, UK) a paper was published yesterday (Malinsky et al., 2015) that was an instant classic. The publication laid down the groundwork necessary to unravel the genetic basis of speciation. Nowadays, many scientists that are focused on cichlids have the following objective: “What is the genetic basis of speciation?” For decades researchers and aquarists alike have debated whether so-called sympatric speciation exists, i.e. can a new species evolve in the presence of the ancestral form. Both parties of the divide (sympatric versus allopatric) tried to find support for their ideas among the cichlids of lakes Malawi, Tanganyika, and/or Victoria because these lakes have so many different species that allopatric (geographically separated populations) speciation would have been virtually impossible to imagine. With the study of the Cameroon and Nicaragua crater lakes which are very small and contain at most a dozen different cichlid species each, it became very convincing that sympatric speciation does indeed exist.
Turner and colleagues went looking for possible sympatric speciation in other crater lakes; in particular those closely associated with Lake Malawi so that results obtained from such crater cichlids could easily be extrapolated to that lake’s huge species flock. They were very successful and found six crater lakes in southern Tanzania that each contained a varying number of species that are all closely related to Malawi cichlids. Malinsky et al. (2015) deal with the two cichlid species they encountered in Lake Massoko, a tiny crater lake with a diameter of about 700 meters and about 35 meters deep. In the shallow waters of the lake, less than 5 meters deep, they found Astatotilapia calliptera or at least a form that closely resembles that species with yellow males and gray-beige females. In the deeper regions (20-25 meters) of the lake, however, they encountered a very similar form but with blue males; its females were indistinguishable from those in shallow water. No yellow males were encountered in deep water. They assumed that these were two different species and examined their DNA to find proof of that. In the course of their investigations they sequenced the whole genome of 146 individuals (from Massoko but also from the neighboring Mbaka River and from Itamba, another crater lake in the area) and although they failed to find a single fixed genetic difference between the two forms they did find 98 “islands” (areas) on the chromosomes that differed between the two forms. They further demonstrated that the blue morph was derived from the yellow, shallow-water morph, and also that the A. calliptera forms of each lake/river are genetically closer to each other than to a population outside their range. They also demonstrated that the Massoko forms are most closely related to the A. calliptera form in the Mbaka River which makes it likely that the Massoko crater lake was initially stocked with fish from that river.
The paper further describes where on the chromosomes the differentiated areas are and what genes these diverged areas hold. Since these chromosome areas are the only places that differ among the two morphs they likely hold the genes that are responsible for the speciation event that has happened or is currently happening. One can imagine various scenarios of how new species evolve with one being a certain beneficial mutation that would allow the owner to survive in a different part of the environment. This would, however, leave a fixed difference in the DNA which could in this case not (yet) be detected. Another possibility could be that some males obtained a different breeding dress which, through sexual selection, could have given rise to a new species. Malinsky et al. (2015) also investigated whether the two Massoko forms were segregated through sexual selection by performing mate choice experiments. This was not that easy to execute because the females of both forms look identical and since they couldn’t find a single fixed genetic difference between the two forms they had to make a combination of various DNA sequences derived from the highly diverged areas of the chromosome. This was successfully tested on the easily distinguishable males, so they could use these probes to tell females of either morph apart. What they then found was surprising. It turned out that the females of the shallow water form were much more selective choosing their mates than those of the blue, deep-water form which would mate with either blue or yellow male. This would mean that if these two forms would be found truly sympatrically (syntopically) the blue form would quickly disappear. A third possible scenario in which a new species can evolve in sympatry is where the ancestral species develops a liking or an adaptation towards a slightly different habitat where it may source a different kind of food. Malinsky et al. (2015) also looked into that possibility and they discovered that indeed there is a difference in diet between the two forms. So it seems that an adaptation event leading to the survival of a subgroup of the ancestral population in the deeper layers of the lake also caused it to speciate (or at least to diverge). Since it is not clear whether the two recognizable forms in Lake Massoko represent true species—the authors refer to them as ecomorphs—or, more likely, are two subpopulations in the process of speciation, they are therefore the ideal candidates to study what genes are initially altered to fix a possible speciation event.
In my opinion such a speciation event is just the result of chance happenings whereby a situation is created in which the two subgroups are actually evolving separately (micro-allopatrically) even when separated by just 50 meters of water. As long as the micro-habitats are not changing the two forms have more time to adapt to their specific environment possibly leading to speciation. The interesting questions are of course what are the genes involved in the initial stage of speciation and which are important to make the event irreversible. I expect a lot more future publications derived from these two cichlid forms in Lake Massoko. The paper is unfortunately not freely downloadable but its supplemental material is. This includes also two video clips showing the two forms underwater in Lake Massoko
- Malinsky, Milan & R.J. Challis, A.M. Tyers, S. Schiffels, Y. Terai, B.P. Ngatunga, E.A. Miska, R. Durbin, M.J. Genner, G.F. Turner. 2015. "Genomic islands of speciation separate cichlid ecomorphs in an East African crater lake". Science. v. 350(n. 6267), pp. 1493-1498 (crc07012) (摘要)
© Copyright 2015 Ad Konings, all rights reserved
Konings, Ad. (十二月 19, 2015). "The Lake Massoko cichlids". Cichlid Room Companion. 檢索 七月 17, 2019, 來自: https://www.cichlidae.com/section.php?id=288&lang=zh.