Molecular Ecology (2007) 16, 2383–2392 doi: 10.1111/j.1365-294X.2007.03322.x © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Ltd Blackwell Publishing Ltd Egg production and individual genetic diversity in lesser kestrels JOAQUÍN ORTEGO, GUSTAU CALABUIG, PEDRO J. CORDERO and JOSÉ MIGUEL APARICIO Grupo de Investigación de la Biodiversidad Genética y Cultural, Instituto de Investigación en Recursos Cinegéticos — IREC (CSIC, UCLM, JCCM), Ronda de Toledo s/n, E-13005 Ciudad Real, Spain Abstract Fecundity is an important component of individual fitness and has major consequences on population dynamics. Despite this, the influence of individual genetic variability on egg production traits is poorly known. Here, we use two microsatellite-based measures, homozygosity by loci and internal relatedness, to analyse the influence of female genotypic variation at 11 highly variable microsatellite loci on both clutch size and egg volume in a wild population of lesser kestrels (Falco naumanni). Genetic diversity was associated with clutch size, with more heterozygous females laying larger clutches, and this effect was statis- tically independent of other nongenetic variables such as female age and laying date, which were also associated with fecundity in this species. However, egg volume was not affected by female heterozygosity, confirming previous studies from pedigree-based breeding experiments which suggest that this trait is scarcely subjected to inbreeding depression. Finally, we explored whether the association between heterozygosity and clutch size was due to a genome-wide effect (general effect) or to single locus heterozygosity (local effect). Two loci showed a stronger influence but the correlation was not fully explained by these two loci alone, suggesting that a main general effect underlies the asso- ciation observed. Overall, our results underscore the importance of individual genetic variation for egg production in wild bird populations, a fact that could have important implications for conservation research and provides insights into the study of clutch size evolution and genetic variability maintenance in natural populations. Keywords: clutch size, egg production, egg size, Falco naumanni, genetic variability maintenance, heterozygosity Received 18 October 2006; revision received 17 January 2007; accepted 7 February 2007 Introduction Mating between closely related parents generally results in progenies with reduced fitness, a fact that has attracted attention since Darwin in several fields of biology and medicine (Wright 1977). This phenomenon, known as inbreeding depression, supposedly occurs because of reduced genome-wide heterozygosity at loci affecting fitness, either increasing the chance of expression of deleterious recessive alleles or reducing the heterozygote advantage for traits that are controlled by genes experien- cing some form of balancing selection (Charlesworth & Charlesworth 1987; Falconer & Mackay 1996). However, the recording of deep pedigrees to estimate inbreeding coefficients is a difficult task to do in open natural populations. Given that the effects of inbreeding depression on fitness occur through a reduction of heterozygosity, an alternative measure for genome-wide heterozygosity is the use of multilocus heterozygosity at neutral polymorphic markers. Many recent studies have reported correlations between individual heterozygosity at neutral microsatellite markers and different key components of fitness (e.g. Foerster et al . 2003; Bean et al . 2004; Hoffman et al . 2004; Charpentier et al . 2005; Acevedo-Whitehouse et al . 2006; see, however, Duarte et al . 2003; Hoffman et al . 2006). However, the utility of this approach has been questioned on the basis of recent empirical and theoretical studies showing that the small number of neutral markers generally employed to estimate genome-wide heterozygosity only correlates weakly with inbreeding coefficients or does so under a restricted number of circumstances (Balloux Correspondence: Joaquín Ortego, Fax: +34 926295 451; E-mail: joaquin.ortego@uclm.es