The house mice of Faray, Orkney

Towards a 'Sea-Level Sensitive' dynamic model: impact of island ontogeny and glacio-eustasy on global patterns of marine island biogeography

A synthetic model is presented to enlarge the evolutionary framework of the General Dynamic Model (GDM) and the Glacial Sensitive Model (GSM) of oceanic island biogeography from the terrestrial to the marine realm. The proposed 'Sea-Level Sensitive' dynamic model (SLS) of marine island biogeography integrates historical and ecological biogeography with patterns of glacio-eustasy, merging concepts from areas as diverse as taxonomy, biogeography, marine biology, volcanology, sedimentology, stratigraphy, palaeontology, geochronology and geomorphology. Fundamental to the SLS model is the dynamic variation of the littoral area of volcanic oceanic islands (defined as the area between the intertidal and the 50-m isobath) in response to sea-level oscillations driven by glacial-interglacial cycles. The following questions are considered by means of this revision: (i) what was the impact of (global) glacio-eustatic sea-level oscillations, particularly those of the Pleistocene glacial-interglacial episodes, on the littoral marine fauna and flora of volcanic oceanic islands? (ii) What are the main factors that explain the present littoral marine biodiversity on volcanic oceanic islands? (iii) How can differences in historical and ecological biogeography be reconciled, from a marine point of view? These questions are addressed by compiling the bathymetry of 11 Atlantic archipelagos/islands to obtain quantitative data regarding changes in the littoral area based on Pleistocene sea-level oscillations, from 150 thousand years ago (ka) to the present. Within the framework of a model sensitive to changing sea levels, we discuss the principal factors affecting the geographical range of marine species; the relationships between modes of larval development, dispersal strategies and geographical range; the relationships between times of speciation, modes of larval development, ecological zonation and geographical range; the influence of sea-surface temperatures and latitude on littoral marine species diversity; the effect of eustatic sea-level changes and their impact on the littoral marine biota; island marine species-area relationships; and finally, the physical effects of island ontogeny and its associated submarine topography and marine substrate on littoral biota. Based on the SLS dynamic model, we offer a number of predictions for tropical, subtropical and temperate volcanic oceanic islands on how rates of immigration, colonization, in-situ speciation, local disappearance, and extinction interact and affect the marine biodiversity around islands during glacials and interglacials, thus allowing future testing of the theory.

Allozymic polymorphism among 14 populations of the house mouse, Mus musculus domesticus, from Greece

Nineteen loci from 239 individuals of the house mouse Mus musculus domesticus (Rodentia, Muridae) were analyzed by means of thin layer electrophoresis. The mice were collected from 14 localities of Greece mainly confined to the area of NW Peloponnese, where a Robertsonian (Rb) system is observed. The individuals were chromosomally characterized by nine diploid numbers, the 2n = 24, 26, 27, 28, 29, 30, 31, 32, and 40. The statistic elaboration revealed that all 14 populations studied were not characterized by cohesive demic structure and high inbreed levels while the gene flow among them has resulted in low levels of genetic differentiation. The resulting values for Nei's genetic distance corresponded to distances known for the level of geographical populations of M. musculus. Wagner's cladogram for the phylogenetic relations between the populations studied implied that it is the diploid number, rather than the geographical factor, that characterizes or dominates each population, which mainly influences the phylogenetic relationships.

Population genetic structure in a Robertsonian race of house mice: evidence from microsatellite polymorphism

Genetic evidence was assessed for inbreeding and population subdivision in a Robertsonian fusion (Rb) race of the western European form of house mouse, Mus musculus domesticus, in central Belgium. Inbreeding, and the factors responsible for subdivision (genetic drift and extinction-recolonization) can theoretically influence the fixation of underdominant Rb variants. The data consisted of allele frequencies of eight microsatellite loci and of the Rb(4.12) and Rb(5.10) chromosomes. Six populations were sampled once, and a seventh was sampled successively over 3 years. No evidence for inbreeding within populations was found. Levels of between-population subdivision were high (theta = 0.15-0.39), and showed no association with either karyotype or geographical distance over 8-60 km. In addition, low values of effective size were found in the successively sampled population (Ne = 5-20). Cases of significant two-locus disequilibria were associated with the most closely linked pair of microsatellite loci (r = 0.15): also consistent with small effective sizes. These results suggest that both the lack of inbreeding, and the combined effects of genetic drift and extinction-recolonization, may promote Rb polymorphism in M. m. domesticus.

Small mammal differentiation on islands

The reason for the distinctiveness of small mammals on islands has traditionally attracted some imaginative story-telling, usually invoking isolation (as a relict) followed by adaptation and/or random genetic changes. Studies of voles on Orkney, long-tailed field mice on the Hebrides and Shetland, and house mice on the Faroe archipelago show that the main factor in differentiating island races from their mainland ancestors is the chance genetic composition of the founding animals. Subsequent change has necessarily to be based on the genes and frequencies carried by this colonizing group. Probably most post-colonization change is adaptive, although possibly limited in extent both by the initial paucity of variation and by the conservative effect of intragenomic interactions. It is probably helpful to recognize that the 'founder effect' or principle commonly invoked in discussions about evolution on islands involves a founder 'event', followed by founder 'selection'. Island differentiation is not necessarily a precursor to speciation, although the wide occurrence of island endemics suggests that founder effects should not be rejected as a driving force initiating speciation. Notwithstanding, island forms provide a valuable 'laboratory' for testing new genetic combinations, a small proportion of which may prove evolutionarily exciting. Only more empirical studies will uncover their evolutionary importance.

Further studies of a staggered hybrid zone in Mus musculus domesticus (the house mouse)

In the extreme north-east of Scotland (near the village of John o'Groats) there is a small karyotypic race of house mouse (2n = 32), characterized by four metacentric chromosomes 4.10, 9.12, 6.13 and 11.14. We present new data on the hybrid zone between this form and the standard race (2n = 40) and show an association between race and habitat. In a transect south of John o'Groats we demonstrate that the clines for arm combinations 4.10 and 9.12 are staggered relative to the clines for 6.13 and 11.14, confirming previous data collected along an east-west transect (Searle, 1991). There are populations within the John o'Groats-standard hybrid zone dominated by individuals with 36 chromosomes (homozygous for 4.10 and 9.12), which may represent a novel karyotypic form that has arisen within the zone. Alternatively the type with 36 chromosomes may have been the progenitor of the John o'Groats race. Additional cytogenetic interest is provided by the occurrence of a homogeneous staining region on one or both copies of chromosome 1 in some mice from the zone.