Speciation on the beach: solving the mystery of the most misidentified marine organism in the world

In our latest author blog, Rui Faria, Ker­stin Johan­nesson, Mark Rav­inet, and Roger But­lin explain how their intens­ive study of an inter­tid­al hybrid zone has shed new light on the pro­cesses of diver­gence and speciation.

The rough peri­winkle, Lit­tor­ina sax­at­il­is, is a world cham­pi­on when it comes to com­plic­at­ing the life of a bio­lo­gist try­ing to clas­si­fy spe­cies. This mar­ine snail spe­cies is widely dis­trib­uted mainly across the Atlantic rocky shores of Europe but is also seen in North Amer­ica. If you ever go to a rocky beach in Europe, it is very likely that rough peri­winkles are close by. Because of the enorm­ous vari­ation in the shape, size and col­our of rough peri­winkles, they have been giv­en more than 100 dif­fer­ent spe­cies or sub­spe­cies names!

Today we con­sider them a single entity but the ques­tion still remains, why does a spe­cies such as Lit­tor­ina sax­at­il­is evolve (and main­tain) so many life-forms? Per­haps we can learn more from this mar­ine snail about how diversity in nature evolves and is sus­tained. This was the main motiv­a­tion for a research pro­gramme that focuses on the Lit­tor­ina sax­at­il­is sys­tem, with the most recent res­ults obtained by Westram et al. (2018) just pub­lished in Evol­u­tion Let­ters.

Lit­tor­ina sax­at­il­is inhab­its the inter­tid­al, an area char­ac­ter­ized by strong diver­gent select­ive pres­sures. In par­tic­u­lar, there are steep envir­on­ment­al trans­itions in wave action and crab-pred­a­tion. These trans­itions (among oth­ers) have been poin­ted out as the main drivers for the evol­u­tion of diver­gent eco­types over short shore dis­tances (<100m) observed with­in sev­er­al sites in Sweden, Spain and UK. The recent study by Westram et al. focuses on a small part of an island rocky shore in Sweden. This island, like many oth­ers in the area, har­bours a pop­u­la­tion of L. sax­at­il­is divided into two eco­types (Crab and Wave).  In boulder fields of the rocky shore, crabs are com­mon and prey heav­ily on rough peri­winkles. Snails from these areas with a thick shell and a nar­row­er aper­ture are more likely to sur­vive, mak­ing up the Crab eco­type. On the wave-swept cliffs of the same shore, stronger wave action but no crabs means that smal­ler snails with a lar­ger aper­ture have a high­er sur­viv­al rate, res­ult­ing in the Wave ecotype.

Along the rocky shore, boulder fields and cliffs trans­ition into one anoth­er and eco­types reg­u­larly hybrid­ize over nar­row (~30m) con­tact zones, where indi­vidu­als with a mix­ture of both Crab and Wave char­ac­ter­ist­ics are gen­er­ated. Hybrid zones are often described as win­dows into the evol­u­tion­ary pro­cess where we can observe oppos­ing forces at play; diver­gent selec­tion pulls pop­u­la­tions apart while gene flow binds them togeth­er. We opened the cur­tains on the win­dow to start look­ing for loci influ­enced by selec­tion, loci that are strong can­did­ates for genes under­ly­ing the out­stand­ing diver­si­fic­a­tion with­in this system.

Snails occur in high dens­ity, are slow mov­ing and fairly col­lab­or­at­ive, so it was pos­sible to sample 600 of them across a 150m tran­sect (from Crab to Wave hab­it­at) in a day. We also took the exact pos­i­tion of each snail in three dimen­sions with a total sta­tion. Sampling was fol­lowed by detailed char­ac­ter­iz­a­tion of each snail for shape, size and col­our phen­o­types in the lab. Tedi­ous work, even for a ded­ic­ated team, but the view from the Tjärnö Mar­ine Labor­at­ory is inspir­ing and we had plenty of oppor­tun­it­ies for “fika” to gal­van­ise us.

We didn’t just go after snail phen­o­types though. Using tar­get cap­ture, we sequenced ~40,000 ran­dom regions of the L. sax­at­il­is gen­ome. Months of bioin­form­at­ics ana­lyses fol­lowed where, bene­fit­ing from well-estab­lished cline the­ory, we were able to identi­fy a set of loci that a clin­al dis­tri­bu­tion of allele fre­quen­cies (a sig­moid change from high fre­quency in one eco­type to low fre­quency in the oth­er). How­ever, it wasn’t that simple, even neut­ral loci can show a clin­al dis­tri­bu­tion due to isol­a­tion by dis­tance. This promp­ted the invit­a­tion of mod­el­lers who came up with a way to sim­u­late clines and to dis­tin­guish the sig­nal of non-neut­ral clines from the stochast­ic noise. This res­ul­ted in the detec­tion of many more loci influ­enced by selec­tion than if we had used a stand­ard meth­od such as an out­lier scan; sur­pris­ingly, this also included loci that are not strongly dif­fer­en­ti­ated between eco­types. One pos­sible explan­a­tion for this is that bal­an­cing selec­tion favours the main­ten­ance of poly­morph­isms with­in each ecotype.

Next we asked, how were loci influ­enced by selec­tion dis­trib­uted across the Lit­tor­ina sax­at­il­is gen­ome? In order to answer this ques­tion, we had to assemble the gen­ome into chro­mo­somes. We took advant­age of tech­no­lo­gic­al advances in DNA sequen­cing and inform­a­tion from lab crosses to build a genet­ic map: after more than a year of metic­u­lous work we were able to put togeth­er the puzzle. Inter­est­ingly, most of the loci influ­enced by selec­tion were clustered in three large gen­om­ic regions. We are now invest­ig­at­ing this fur­ther, how­ever it is pos­sible that the three regions cor­res­pond to chro­mo­somal rearrange­ments, such as inver­sions. These are known to reduce recom­bin­a­tion and pre­vent the break up of adapt­ive com­bin­a­tions of alleles, facil­it­at­ing the evol­u­tion of eco­types and species.

Our next goal is to determ­ine wheth­er the same gen­ome regions are involved in the evol­u­tion of oth­er L. sax­at­il­is forms. To do this, we are per­form­ing sim­il­ar ana­lyses in oth­er shores of Sweden, UK, France and Spain. Togeth­er with a com­par­is­on to oth­er closely-related mar­ine snail spe­cies, we hope soon to shed light on the mys­tery sur­round­ing this world cham­pi­on in phen­o­typ­ic variation.

The next time you go to a rocky beach, pay atten­tion to the out­stand­ing diversity and remem­ber that a nat­ur­al labor­at­ory for study­ing adapt­a­tion and spe­ci­ation might be right by your feet!

Sev­er­al co-authors of the study con­trib­uted to this blog: Rui Faria, Ker­stin Johan­nesson, Mark Rav­inet, and Roger But­lin. The ori­gin­al study is freely avail­able to read and down­load from Evol­u­tion Let­ters.