RESEARCH ARTICLE Larval fitness and immunogenetic diversity in chytrid-infected and uninfected natterjack toad (Bufo calamita) populations S. May • I. Zeisset • Trevor J. C. Beebee Received: 20 October 2010 / Accepted: 14 January 2011 / Published online: 28 January 2011 Ó Springer Science+Business Media B.V. 2011 Abstract Chytrid fungus Batrachochytrium dendrobati- dis (Bd), an emerging disease, has been decimating amphibian populations around the world for several dec- ades. We quantified aspects of larval fitness, adaptive (major histocompatibility complex) diversity and neutral (microsatellite) diversity in natterjack toad (Bufo calamita) populations in two regions of north-west England. Toads in region one had no evidence of chytrid infection, whereas in region two there was a substantial prevalence of Bd. Larval fitness (growth rate, time to metamorphosis and survival) of B. calamita did not differ between the regions. Genetic diversity at microsatellite loci was much higher in the infected than in the uninfected region, but the converse was true of MHC diversity indicating that genetic drift was unlikely to explain the differences in MHC between the regions. Furthermore, MHC allele frequencies varied sig- nificantly between Bd-infected and uninfected populations. Microsatellite diversity was not a robust indicator of larval fitness in these toad populations while MHC genotype frequencies varied in a way that was consistent with directional selection in response to pathogen prevalence. The acquired immune defences may therefore play an important role in determining the susceptibility of amphibian species to chytridiomycosis. Keywords Amphibian  MHC  Chytrid  Microsatellites  Fitness Introduction Amphibians constitute a biodiverse vertebrate group with over 6,000 species worldwide but together with reptiles they are currently experiencing higher rates of global decline and extinction than mammals or birds (Stuart et al. 2004; Beebee and Griffiths 2005; Beebee et al. 2009). Habitat destruction and alteration are the main causes of decline but an emerging pathogen, the chytrid fungus Batrachochytrium dendrobatidis (Bd), has also had a major impact especially in tropical America and northern Aus- tralia (e.g. Fisher et al. 2009). In Europe amphibian declines started earlier than in most other continents and were especially severe in the 1960s, largely due to habitat loss or damage (Houlahan et al. 2000). However, Bd was discovered in Europe recently and its associated disease, chytridiomycosis, has precipitated declines of two amphibian species in parts of Iberia (Walker et al. 2010). Bd has also been present in Britain as an infection of several amphibian species since at least 2004 (Cunningham et al. 2005). Natterjack toads (Bufo calamita) are rare and fully protected in Britain and occur mostly on coastal dune and upper saltmarsh habitats (Beebee 1977). Recently Bd has appeared in a few B. calamita populations in north-west England (Cunningham and Minting 2008) although as yet there is no evidence of association with population declines. One of the smallest and most isolated natterjack populations, in eastern England, has very low diversity at microsatellite loci and reduced fitness with respect to larval growth and survival relative to other more outbred S. May  I. Zeisset  T. J. C. Beebee (&) School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, UK e-mail: t.j.c.beebee@sussex.ac.uk Present Address: S. May Centre for Infections, Health Protection Agency, 61 Colindale Avenue, London NW9 5EQ, UK 123 Conserv Genet (2011) 12:805–811 DOI 10.1007/s10592-011-0187-z