Wednesday, 28 October 2015

Galapagos Marine Iguana: Genes and Islands

The Galapagos never cease to fascinate. New discoveries are being made all the time on the animals that live there and how they have evolved. 

Marine Iguana. Santa Fé

I watched Marine Iguanas (Amblyrhynchus cristatus) for hours when we there in 2012: lekking behaviour, males fighting, mating, salt glands in action, feeding on algae, and just sitting around on rocks in large numbers.

Marine Iguanas vary in appearance between the islands and those with an obsession for artificial pigeonholes have created subspecies accordingly. I shall ignore those dated divisions and concentrate on a recent paper* in which mitochondrial and nuclear DNA sequences have been used to investigate the population genetic structure.

This paper has highlighted the tendency towards the splitting into different species—speciation—and the hybridization between lineages that tends to wipe out that tendency. While the split between the Galapagos Land Iguanas and the Marine Iguana was calculated to have occurred 4.5 million years ago, division into genetic lineages in the Marine Iguana was found to be very recent—within the last 50,000 years.

As well as differences between islands, different lineages were even found on one island: San Cristobal or Chatham as it was known in Darwin’s day, and to some of us still is. It is presumed that volcanic events, with their huge lava fields divided the populations at some stage; there was a major eruption 1800-3000 years ago which reduced the population and produced a severe bottleneck in both lineages. What is surprising is that migration and hybridization between these two populations, termed LO (for Lobería in the south-west of the island) and PP (for Punta Pitt in the north-east) was uncommon despite there being only 12 km of coastline between them. However, evidence was obtained for hybridization of both LO and PP lineages with animals from other islands rather than the adjacent population. Indeed, migrants were found from Santa Cruz (Indefatigable) and Espaniola (Hood) during the collection of samples. Therefore, introgression of genes from other populations acts to oppose the isolation that would tend towards speciation.

Marine iguana population clusters and phylogenetic relationships. (a) Map of the Galápagos archipelago with major islands colour-coded according to their marine iguana population cluster assignment inferred from structure analysis of 614 individuals genotyped for 12 microsatellite loci (vertical panel in (b)). (b) Species tree cloudogram based on an analysis of 6257 RADSeq-derived SNPs in 33 marine iguanas from across the archipelago, including both San Cristóbal lineages. The graph shows the posterior distribution of consensus trees. Asterisks mark nodes with posterior probability = 1.0 (all other nodes less than 0.9). Specimens were grouped according to population assignment based on structure analysis. From MacLeod et al. 2015

Genetic and morphological differentiation of LO and PP lineages on San Cristóbal Island. LO-SRL and PP-SRPC refer to the original Lobería and Punta Pitt localities, photos show adult LO and PP males. (a) Assignment of 454 individuals based on 18 microsatellite loci, after exclusion of inter-island hybrids and migrants. Abbreviations show sampling locations and 1993 marks specimens sampled in that year. (b) Haplotype network of control region sequences (mtDNA) for LO and PP specimens. (c) Map of sampling localities; arrows indicate migrants/hybrids from Santa Cruz (green), Española (orange) and Lobería (blue); dagger symbols denote locations of within-island hybrids between PP and LO; triangles denote locations of inter-island hybrids. Population SRECA contains Española migrants/hybrids only. Shaded areas mark lava groups 4–6 aged less than 0.1 Ma [40]. (c) Mean, standard deviation and range of morphological variables differing between LO and PP. ***p < 0.001, **p < 0.01, *p < 0.05; sample sizes above each plot, details and abbreviations in Results and electronic supplementary material. From MacLeod et al. 2015

There are differences in size between lineages, PP for example, being smaller than LO, but I will leave consideration of the significance of that and to what happens during El Niño oscillations, when food in the form of algae is short and the populations of Marine Iguana crash, to another time.

We only visited the south of Cristobal, so saw just those of the LO lineage. The question then, of course, is: which ones did Darwin see when he collected on Chatham (named, like Punta Pitt, after William Pitt the Elder, 1st Earl of Chatham, 1708-1778)? LO, certainly, in the south where there are groups of iguanas around Cerro Brujo, for example, but it is likely that he also saw PP, since some of his landing and collection points were well to the north and within the PP lineage’s range.

The photographs shows Marine Iguanas from some of the islands we visited:

Española (Hood)
South Plaza
Isabella (Albemarle)
Fernandina (Narborough)
The extent of the lava fields can be seen in this photograph taken on Isabella between from near Darwin Volcano to the south east:

A close-up of the lava on Santiago (James):

…and these are three videos, all taken on a wonderful morning on Fernandina (Narborough):

*MacLeod A, Rodríguez A, Vences M, Orazco-terWengel P, García C, Reillmich F, Gentile G, Caccone A, Quezada G, Steinfartz S. Hybridization masks speciation in the evolutionary history of the Galapagos marine iguana. Proceedings of the Royal Society. B 282, 20150425.

†Estes G, Grant KT, Grant PR. 2000. Darwin in Galapagos: his footsteps through the archipelago. Notes and Records of the Royal Society of London 54, 343-368.