Mechanisms of Kin Recognition Flashcards
Recognition systems are of fundamental importance at every level of biological organisation
Genetics, individual, group
Adaptive decisions about mate choice, cooperative investment, social affiliation, etc depend on discrimination ability, hence recognition system
Kin recognition – differential treatment of conspecifics differing in genetic relatedness
Theory - actor compares cue against template
undesirable recipients
(NON-KIN)
desirable recipients (KIN)
Recognition errors are inevitable and their frequency depends on relative costs and benefits
Kin recognition does not require genetic cues of kinship – cues are usually environmental
Errors are likely even in kin-selected systems
Mechanism and effectiveness varies depending on selection pressure and context
Recognition systems play a vital role at every level of biological organization, from gene transcription and immune systems within individuals, via identification of food, mates and species, through to the sophisticated communication networks that characterize our own societies
. Are there common principles that govern their evolution? In this lecture we will address this question in the context of kin recognition, demonstrating that there are indeed common principles but also that the strength of selection on active kin discrimination and the mechanism through which it is achieved varies widely.
We follow Sherman et al. (1997) in defining kin recognition as the differential treatment of conspecifics differing in their genetic relatedness. This does not require any specific mechanism (e.g. that it must involve genetic cues, as advocated by Grafen 1990), instead focusing on the outcome, i.e. differential treatment in relation to kinship.
Any indirect fitness benefit of a kin-selected behaviour may be maximized by effective discrimination of kin from non-kin, but it is also important to bear in mind that the costs and benefits of discrimination will vary according to its ecological and evolutionary context, and may have diverse outcomes.
Components of recognition systems and the Acceptance Threshold Model
Any recognition process involves an actor and recipient, and there are three components of a recognition system:
(i) Production component – the cues (labels) in recipients that allows actors to recognize them.
(ii) Perception component – the sensory detection of cues by actors and subsequent phenotype matching of that cue to a template of desirable (fitness-enhancing) or undesirable (fitness-reducing) recipients.
(iii) Action component – the action performed that depends on the similarity between the actor’s template and the recipient’s cue.
The acceptance threshold model (Reeve (1989) American Naturalist 133: 407-435) argues that the cues of desirable and undesirable recipients are likely to overlap, and that actors should have a threshold for acceptance/rejection that optimizes the balance of accepting undesirable recipients and rejecting desirable ones (Figure 1).
A ‘generous’ strategy is predicted when the cost:benefit ratio is low, but a ‘conservative’ strategy when that ratio is high. There is good, but limited, evidence for the acceptance threshold model. For example, in honeybees colony guards adjust their rejection/acceptance of bees trying to enter the colony according to the risk of nectar robbery (Downs & Ratnieks (2000) Behavioral Ecology 11: 326-333; Couvillon et al. (2008) Animal Behaviour 76: 1653-1658).
Mechanisms of recognition
In principle, the production, perception and action components of any recognition system could be determined either genetically or environmentally. Here, we examine the empirical evidence for genetic and learned cues to kinship.
Genetic cues to kinship
‘Greenbeard genes’ – These were postulated by Dawkins (1976) in The Selfish Gene as recognition alleles that signal themselves, recognize themselves in other individuals, and direct cooperation to other bearers of that gene. These multiple functions seem an unlikely property of a single gene, plus ‘greenbeards’ are vulnerable to exploitation by ‘falsebeards’ that signal but don’t cooperate, so it is unsurprising that examples of ‘greenbeards’ are very rare. One putative example, in the fire ant Solenopsis invicta (Keller & Ross (1998) Nature 394: 573-575) actually turns out to be a large section (55%) of a ‘social chromosome’ encompassing 616 genes that do not experience the disruption of recombination due to a chromosomal inversion (Wang et al. (2013) Nature 493: 664-668).
Markers indicating gene-sharing – This was dubbed the ‘armpit effect’ by Dawkins (1976), and postulates that comparison of genetically-determined cues versus a template via self-inspection allows actors to assess relatedness. MHC genes are good candidates because they are highly polymorphic, can be detected through scent, and tend to be inherited as ‘types’. Tests of discrimination among familiar siblings indicate preference for the same MHC type in arctic char (Olsen et al. (2002) Journal of Chemical Ecology 28: 783-795) and Xenopus laevis (Villinger & Waldman (2008) Proceedings of the Royal Society B 275: 1225-1230), but in neither case was discrimination among unfamiliar recipients tested. The best evidence comes from a recent study of nest partner preference in house mice (Green et al. (2015) Current Biology 25: 2631-2641). Genes encoding MUPs (mouse urinary proteins, a species-specific kinship marker) are inherited as tight linkage units, like MHC haplotypes, and females prefer to nest with females that share their MUP genotype, whether familiar or unfamiliar. These results indicate a process of ‘self-referent phenotype matching’, i.e. discriminating in favour of conspecifics that have a phenotype that matches your own.
Environmental cues to kinship
A ‘rule of thumb’ based on some environmental cue may provide a simpler mechanism for effective kin discrimination.
Spatial cues – A rule such as ‘feed anything in my own nest or territory’ may be an effective means of discriminating between one’s own offspring and unrelated young.
However, such a rule may be exploited by intra- or inter-specific brood parasites (see Manipulation lecture), and it only works when the risk of misdirected care is low. For example, in colonial bank swallows, there is no discrimination of nestlings until a few days before fledging, but shortly before they leave the nest offspring develop signature calls that can be recognized by parents (Beecher et al. (1981) Animal Behaviour 29: 95-101).
Learned cues – Rules such as ‘treat anyone I was reared with as kin’, or ‘treat as kin anyone whose sound/smell/ appearance is familiar’ appear to be a widespread mechanism of kin recognition. For example, honeybees use environmentally acquired and learned colony odours for discrimination, and humans avoid close childhood associates as future sexual partners. Experimental tests of learning as a mechanism for kin recognition using cross-fostering of nestlings have been conducted in long-tailed tits (Sharp et al. (2005) Nature 434: 1127-1130) and Seychelles warblers (Komdeur et al. (2004) Proceedings of the Royal Society B 271: 963-969).
Conclusions
An important point to remember is that active kin recognition and discrimination will evolve only when it is adaptive. For example, there is great variation among cooperative breeders in whether helpers exhibit discrimination (Griffin & West (2003) Science 302: 634-636) and this variation is correlated with the probability of making errors (Cornwallis et al. (2009) Journal of Evolutionary Biology 23: 738-747).
In some species, where helpers are philopatric in stable groups on stable territories, any young present are likely to be close kin, so there is no need for active kin discrimination – a rule of feeding any young on the natal territory suffices. In species, such as the long-tailed tit, where there is a substantial risk of helping to feed non-kin, helpers exhibit strong kin discrimination in order to maximize the indirect fitness benefits that they can receive. This means that, contrary to some claims in the literature, even in kin-selected systems we should not necessarily expect to see evidence for active kin discrimination. The strength of selection for kin recognition, and the mechanism through which it is achieved will vary greatly across taxa in relation to the ecological and social contexts experienced.
