Inbreeding parents should invest more resources in fewer offspring

Abstract

Inbreeding increases parent–offspring relatedness and commonly reduces offspring viability, shaping selection on reproductive interactions involving relatives and associated parental investment (PI). Nevertheless, theories predicting selection for inbreeding versus inbreeding avoidance and selection for optimal PI have only been considered separately, precluding prediction of optimal PI and associated reproductive strategy given inbreeding. We unify inbreeding and PI theory, demonstrating that optimal PI increases when a female's inbreeding decreases the viability of her offspring. Inbreeding females should therefore produce fewer offspring due to the fundamental trade-off between offspring number and PI. Accordingly, selection for inbreeding versus inbreeding avoidance changes when females can adjust PI with the degree that they inbreed. By contrast, optimal PI does not depend on whether a focal female is herself inbred. However, inbreeding causes optimal PI to increase given strict monogamy and associated biparental investment compared with female-only investment. Our model implies that understanding evolutionary dynamics of inbreeding strategy, inbreeding depression, and PI requires joint consideration of the expression of each in relation to the other. Overall, we demonstrate that existing PI and inbreeding theories represent special cases of a more general theory, implying that intrinsic links between inbreeding and PI affect evolution of behaviour and intrafamilial conflict.

Differential effects of offspring and maternal inbreeding on egg laying and offspring performance in the burying beetleNicrophorus vespilloides

“…We also found that females that were mated to related males (and thus were producing inbred offspring) laid fewer eggs than those that were mated to unrelated males (and thus producing outbred offspring). This finding is consistent with a recent theoretical model predicting that parents producing inbred offspring should produce fewer offspring and invest more resources in individual offspring (Duthie et al ., ). Additionally, we found that females producing highly inbred offspring took longer to begin egg laying after encountering the carcass.…”

“…There is evidence that maternal care buffers against the detrimental effect of offspring inbreeding in N. vespilloides (Pilakouta et al ., ). Furthermore, the smaller clutch sizes laid by females producing inbred offspring may lead to a lower initial number of larvae hatching, which would allow females to provide more care to each larva in inbred broods as predicted by a recent theoretical model (Duthie et al ., ). Thus, nonlinear effects of inbreeding could arise if females overcompensate for the detrimental effects of moderate offspring inbreeding by providing more care, resulting in higher larval survival for moderately inbred offspring compared to outbred offspring.…”

Producing offspring in Armadillidium vulgare: Effects of genetic diversity and inbreeding

“…It is likely that the homozygous state of loci with deleterious recessive alleles leads to a decrease in inbred offspring fitness (Charlesworth & Willis, 2009). Another hypothesis is that a female mating with a relative should increase her parental investment in the inbred offspring, therefore reducing their number (Duthie, Lee, & Reid, 2016). So far the possibility that parents decrease their investment in these broods to limit energetic expenditure in producing bad quality homozygous offspring has received little attention (Bilde, Maklakov, & Schilling, 2007).…”

Plant and Animal Reproductive Strategies: Lessons from Offspring Size and Number Tradeoffs

“…Thus far, experimental evidence for the adaptive significance of variation in egg size across different conditions is limited, possibly because experimental conditions do not adequately mimic the complexity of natural environments. Other biotic factors that strongly select for large or small differential offspring size include food quality (e.g., Brody and Lawlor, 1984;Braby, 1994), competition (e.g., Parker and Begon, 1986), inbreeding (e.g., Duthie et al, 2016), limited dispersal (e.g., Kuijper and Johnstone, 2012) and predation (e.g., Kerfoot, 1974;Ernsting and Isaaks, 1997) (see also Table 2 in Rollinson and Rowe, 2016). In particular, increased competition generally selects for larger offspring size, while sizedependent predation may select for either smaller or larger offspring.…”