Inbreeding and Its Fitness Effects in an Insular Population of Song Sparrows (Melospiza melodia)

Abstract

Inbreeding depression is thought to be a major factor affecting the evolution of mating systems and dispersal. While there is ample evidence for inbreeding depression in captivity, it has rarely been documented in natural populations. In this study, I examine data from a long-term demographic study of an insular population of song sparrows (Melospiza melodia) and present evidence for inbreeding depression. Forty-four percent of all matings on Mandarte Island, British Columbia, were among known relatives. Offspring of a full-sib mating (f = 0.25) experienced a reduction in annual survival rate of 17.5% on average. Over their lifetime, females with f = 0.25 produced 48% fewer young that reached independence from parental care. In contrast, male lifetime reproductive success was not affected by inbreeding. Reduced female lifetime reproductive success was mostly due to reduced hatching rates of the eggs of inbred females. Relatedness among the parents did not affect their reproductive success. Using data on survival from egg stage to breeding age, I estimated the average song sparrow egg on Mandarte Island to carry a minimum of 5.38 lethal equivalents (the number of deleterious genes whose cumulative effect is equivalent to one lethal); 2.88 of these lethal equivalents were expressed from egg stage to independence of parental care. This estimate is higher than most estimates reported for laboratory populations and lower than those reported for zoo populations. Hence, the costs of inbreeding in this population were substantial and slightly above those expected from laboratory studies. Variability in estimates of lethal equivalents among years showed that costs of inbreeding were not constant across years.

Sensitive males: inbreeding depression in an endangered bird

“…If highly inbred females are dying at the very early embryo stage beyond the detection capability of this study, then this may suggest a direct physiological mechanism in which the heterogametic sex is more vulnerable owing to sexlinked recessive genes (Haldane 1922). Sex-specific effects of inbreeding have previously been found in two wild avian populations: the Mandarte Island song sparrow (Melospiza melodia; Keller 1998;Reid et al 2007) and New Zealand takahe (P. mantelli; Jamieson et al 2003). Song sparrow (Keller 1998) and takahe (Jamieson et al 2003) show female-specific effects of inbreeding on reproduction, while in the same population of song sparrows, the direction of sex-specific effects differs among components of immunity (Reid et al 2007).…”

“…Therefore, when comparing with other studies that use this approach, hihi have a moderate to high genetic load (e.g. song sparrow (M. melodia), Keller 1998; collared flycatcher (Ficedula albicollis), Kruuk et al 2002;Darwin finches, Keller et al 2002; great tit (Parus major), Szulkin & Sheldon 2007) and is also higher than most estimates reported for laboratory populations, and lower than those reported for zoo populations (for reviews see Lynch & Walsh 1998;Keller 1998).…”

“…Sex-specific effects of inbreeding have previously been found in two wild avian populations: the Mandarte Island song sparrow (Melospiza melodia; Keller 1998;Reid et al 2007) and New Zealand takahe (P. mantelli; Jamieson et al 2003). Song sparrow (Keller 1998) and takahe (Jamieson et al 2003) show female-specific effects of inbreeding on reproduction, while in the same population of song sparrows, the direction of sex-specific effects differs among components of immunity (Reid et al 2007). These examples also show that sex-specific inbreeding effects may differ depending on the life-history trait measured.…”

Towards modelling persistence of woodland birds: the role of genetics

“…Harmful effects of inbreeding are virtually ubiquitous, and cause major fitness and demographic declines in free-living populations of birds (Keller et al 1994;Keller 1998;Markert et al 2004). Important functions including disease-resistance, reproduction and population recruitment are damaged disproportionately by inbreeding (Keller 1998;Reid et al 2003).…”

“…Harmful effects of inbreeding are virtually ubiquitous, and cause major fitness and demographic declines in free-living populations of birds (Keller et al 1994;Keller 1998;Markert et al 2004). Important functions including disease-resistance, reproduction and population recruitment are damaged disproportionately by inbreeding (Keller 1998;Reid et al 2003). It is thus unsurprising that inbreeding through population contraction leads to increased extinction risk (Saccheri et al 1998;Nieminen et al 2001;Frankham 2005a;O'Grady et al 2006).…”