By: Claire Shamber, Lauren O’Regan, Julia Nuzzo, and Julia Tawil (Stonehill College, BIO323: Evolution, Spring 2020)
Periwinkle is not just a color; it is also an organism! Flat periwinkles are small sea snails that live throughout the northern shores of the Atlantic Ocean. Researchers have been interested in periwinkles because of their large dispersal across different shores and their potential to provide information about the intertidal zone. The intertidal zone is the area where the ocean meets the shoreline between low and high tides. This environment is subject to ecological speciation, meaning that the varying conditions of the intertidal zone can create differences between species. Speciation is the process of a single species splitting into two separate distinct species. Periwinkles offer a good look into how we determine whether two populations should be considered as their own species.
There are specifically two periwinkle sister species that are ideal representatives of possible speciation. Why do scientists claim that these species, Littorina fabalis and Littorina obtusata, are sister species? L. fabalis and L. obtusata are distributed similarly throughout the oceans, are structurally similar, and come from a common ancestral species. However, the two differ in terms of how they relate to one another as well as their relationships to their environment. The existence of the similarities leads scientists to think that L. fabalis and L. obtusata are sister species.
Although there is information known about their structures and their relationship with the environment, there has been limited research on whether genetics supports the sister species relationship and how the split occurred. Due to these lingering questions, researchers decided to focus on these species in order to answer even bigger questions about the overall speciation of the intertidal zone. Currently, one of the most accurate methods for providing evidence of speciation is to look at the genetics of each sister species. Knowing the traits of the two species allows for comparison of genes between populations which provides insight into their relationship. In studying the genetics of L. fabalis and L. obtusata, researchers looked for shared genes as well as genetic diversity to determine if the two species were previously one species that diverged into two. They also observed geographical aspects of the two species looking at barriers to migration, the effect of glacial periods on species dispersal, and the influence of geography on genetic differentiation in hopes to gain insight on the speciation within the intertidal zone.
In order to observe the genetic components of L. fabalis and L. obtusata, multiple genes were chosen to be compared in order to strengthen results. Therefore, researchers investigated two genes from two different locations within the periwinkle cells (nucleus and mitochondria) among 102 total individuals. After isolating and analyzing the two genes from L. fabalis and L. obtusata, they developed a visual representation of the relationships between individuals, a phylogeny. The final version of the phylogeny showed five major groupings of individuals, which corresponded to two groups of L. obtusata and three groups of L. fabalis. Within these five groups, there were secondary groups that often correlated to geographical location within the species.
In order to arrive at the final phylogeny, the researchers sampled 211 L. fabalis and 161 L. obtusata from the northeast Atlantic coast. DNA was taken from the head and foot tissue to analyze the genetic traits of each sample. A method called polymerase chain reaction (PCR) was used to amplify and replicate both mitochondrial and nuclear DNA. A PCR machine allows scientists to work with even small amounts of DNA because it simulates optimal conditions for copying DNA samples until there is an abundance. Both the nuclear and mitochondrial DNA were sequenced and organized to construct trees showing the relationship and an estimation of divergence time between the two flat periwinkle species.
From this genetic data, the authors were able to confirm that an exchange of alleles had occurred between L. fabalis and L. obtusata. The results showed that both species prefer larger females and that mating between the two species had occurred. There were three genes inherited together from a single parent present in the L. obtusata individuals that originated from central Europe that were grouped with genes that were common in L. fabalis species. This served as supporting evidence that there was an alteration of genes between the two different species. Further research needs to be done because it is unknown if this gene flow could have occurred in the UK before the migration to the north. In order to get a better look at the modification of the genes, scientists looked at gene flow using a program called IMa2. This program was able to confirm that a transfer of genes had occurred between the two species and that mating took place between them on the Iberian Peninsula and in northern central Europe. The transfer of genes was higher in areas where the two species shared the same geographical area. During the study, it was seen that there was a strong barrier between the two regions that the snail was taken from.
This study was able to show that there was divergence with gene flow between the two sister species. The exchange of genes between the populations did occur, but scientists were not able to determine the direction. There was also a lower genetic diversity in the Iberian population compared to the northern central populations. To further investigate the intertidal zone, it would be ideal to conduct experiments on other organisms and possible sister species such as sponges and sea stars.
Article: Sotelo, G., M. Duvetorp, D. Costa, M. Panova, K. Johannesson, and R. Faria. 2020. Phylogeographic history of flat periwinkles, Littorina fabalis and L. obtusata. BMC Evolutionary Biology 20: 23.