Characterizing Source–sink Dynamics With Genetic Parentage Assignments

Abstract

Source-sink dynamics have been suggested to characterize the population structure of many species, but the prevalence of source-sink systems in nature is uncertain because of inherent challenges in estimating migration rates among populations. Migration rates are often difficult to estimate directly with demographic methods, and indirect genetic methods are subject to a variety of assumptions that are difficult to meet or to apply to evolutionary timescales. Furthermore, such methods cannot be rigorously applied to high-gene-flow species. Here, we employ genetic parentage assignments in conjunction with demographic simulations to infer the level of immigration into a putative sink population. We use individual-based demographic models to estimate expected distributions of parent-offspring dyads under competing sink and closed-population models. By comparing the actual number of parent-offspring dyads (identified from multilocus genetic profiles) in a random sample of individuals taken from a population to expectations under these two contrasting demographic models, it is possible to estimate the rate of immigration and test hypotheses related to the role of immigration on population processes on an ecological timescale. The difference in the expected number of parent-offspring dyads between the two population models was greatest when immigration into the sink population was high, indicating that unlike traditional population genetic inference models, the highest degree of statistical power is achieved for the approach presented here when migration rates are high. We used the proposed genetic parentage approach to demonstrate that a threatened population of Marbled Murrelets (Braclhyrarmphus marmotus) appears to be supplemented by a low level of immigration (approximately 2-6% annually) from other populations.

Towards modelling persistence of woodland birds: the role of genetics

“…linkage disequilibrium). For many methods (but not all; Peery et al 2008), migrants can be most easily detected in strongly differentiated populations, because when migrants are numerous, residents and migrants become more genotypically similar, making them harder to distinguish Manel et al 2005). Detection of immigrants where all sources are sampled can be conducted with BayesAss+ and GeneClass2, whereas detection of immigrants from sources that may be inferred is the focus of BAPS, NewHybrids, Structure, Geneland, BATWING, IM/IMa, LAMARC and MSVAR (reviewed and summarised in Excoffier and Heckel 2006).…”

“…Recent approaches present potential solutions to some of these challenges. Source-sink dynamics may be important in wildlife and especially in landscapes affected by human activity, but information is limited because of the many issues associated with estimating rates of migration among units by field methods, and the limitations of indirect genetic approaches (Peery et al 2008). One largely untapped source of information is the application of parentage assignments to assist with inferring immigration into candidate sink populations.…”

“…One largely untapped source of information is the application of parentage assignments to assist with inferring immigration into candidate sink populations. Peery et al (2008) developed individual-based simulations to model expected distributions of parent-offspring pairs under competing demographic models. These calibrations were used to interpret the observed real-world genetic data, and ascribe relative support for competing demographic models.…”

Genetic analyses of historic and modern marbled murrelets suggest decoupling of migration and gene flow after habitat fragmentation

“…A significantly lower proportion of murrelets classified as migrants in central California were involved in parentoffspring pairs (2 of 19 or 10.5%) than residents (114 of 251 or 45.4%, x 2 1 ¼ 8:78, p , 0.003) in the modern sample, indicating that migrants contributed fewer offspring to this population than residents. Two migrants possessed a parent or offspring in the population, but approximately two residents were expected to be mistakenly classified as migrants and about 10 per cent of putative parent-offspring dyads were probably type I errors (Peery et al 2008a;Hall et al 2009). Thus, it is possible that no migrant actually possessed a parent or offspring in the sample.…”

“…Pairs of individuals sharing at least one allele at all loci were considered to be probable parent -offspring dyads. In a previous study, we used Monte Carlo simulations to demonstrate that less than 10 per cent of putative parent -offspring dyads share an allele at all loci by chance and represent 'false matches' given the 16 microsatellite loci considered here (Peery et al 2008a). …”