Evolution of defences against cuckoo (Cuculus canorus) parasitism in bramblings (Fringilla montifringilla): a comparison of four populations in Fennoscandia

The complete mitochondrial genome of the Brambling Fringilla montifringilla (Passeriformes: Fringillidae) from Maor Mountain, China

The Brambling (Fringilla montifringilla) (Linnaeus 1758) is a member of the Passeriformes family of birds and primarily feeds on grass seeds and grains. Muscle tissue was collected from birds sampled from Moar Mountain, China, and the complete mitochondrial genome was sequenced. Its mitochondrial genome consists of 13 protein-coding genes (PCGs), 2 rRNA genes (12S rRNA and 16S rRNA), 22 tRNA genes, and 1 control region (CR). The genome comprises 30.30% A, 23.32% T, 14.31% G, and 32.07% C bases. Phylogenetically, F. montifringilla is closely related to the Fringilla coelebs, Fringilla teydea teydea and Fringilla polatzeki.

Avian egg and nestling detection in the wild: should we rely on visual models or behavioural experiments?

The fields of avian egg and nestling colour detection have rapidly advanced owing to the application of visual models, which have allowed assessing of evolutionary questions considering receiver perception. Here, I first review the literature aiming to identify patterns of avian visual model usage. Second, I elaborate on limitations in the application of the receptor-noise limited perceptual (RNL hereafter) model. A systematic literature review revealed that the RNL model was the most used approach (81% of studies) in the field, and that most studies (76%) were concerned with classic evolutionary questions in avian brood parasitism. Some known limitations of the RNL model deal with model assumptions and parameterization, or, a poor consideration of post-detection neural processes. Others, however, are specific of the fields of egg and nestling discrimination and deal with the highly variable nature of ambient light at the nests, the complex colour design of eggs and nestlings, the multi-dimensional nature of perception, and the possible implication of learning. I, therefore, conclude that visual models should be used with caution to establish inference about egg and nestling discrimination, and rather be used to provide reasonable hypotheses which need to be validated with behavioural experiments. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Long-term coevolution between avian brood parasites and their hosts

Coevolutionary theory predicts that the most common long-term outcome of the relationships between brood parasites and their hosts should be coevolutionary cycles based on a dynamic change selecting the currently least-defended host species, given that when well-defended hosts are abandoned, hosts will be selected to decrease their defences as these are usually assumed to be costly. This is assumed to be the case also in brood parasite-host systems. Here I examine the frequency of the three potential long-term outcomes of brood parasite-host coevolution (coevolutionary cycles, lack of rejection, and successful resistance) in 182 host species. The results of simple exploratory comparisons show that coevolutionary cycles are very scarce while the lack of rejection and successful resistance, which are considered evolutionary enigmas, are much more frequent. I discuss these results considering (i) the importance of different host defences at all stages of the breeding cycle, (ii) the role of phenotypic plasticity in long-term coevolution, and (iii) the evolutionary history of host selection. I suggest that in purely antagonistic coevolutionary interactions, such as those involving brood parasites and their hosts, that although cycles will exist during an intermediate phase of the interactions, the arms race will end with the extinction of the host or with the host acquiring successful resistance. As evolutionary time passes, this resistance will force brood parasites to use previously less suitable host species. Furthermore, I present a model that represents the long-term trajectories and outcomes of coevolutionary interactions between brood parasites and their hosts with respect to the evolution of egg-rejection defence. This model suggests that as an increasing number of species acquire successful resistance, other unparasitized host species become more profitable and their parasitism rate and the costs imposed by brood parasitism at the population level will increase, selecting for the evolution of host defences. This means that although acceptance is adaptive when the parasitism rate and the costs of parasitism are very low, this cannot be considered to represent an evolutionary equilibrium, as conventional theory has done to date, because it is not stable.

Ejecting chick cheats: a changing paradigm?

Evolutionary arms-races between avian brood parasites and their hosts have typically resulted in some spectacular adaptations, namely remarkable host ability to recognize and reject alien eggs and, in turn, sophisticated parasite egg mimicry. In a striking contrast to hosts sometimes rejecting even highly mimetic eggs, the same species typically fail to discriminate against highly dissimilar parasite chicks. Understanding of this enigma is still hampered by the rarity of empirical tests - and consequently evidence - for chick discrimination. Recent work on Australian host-parasite systems (Gerygone hosts vs. Chalcites parasites), increased not only the diversity of hosts showing chick discrimination, but also discovered an entirely novel host behavioural adaptation. The hosts do not desert parasite chicks (as in all previously reported empirical work) but physically remove living parasites from their nests. Here, I briefly discuss these exciting findings and put them in the context of recent empirical and theoretical work on parasite chick discrimination. Finally, I review factors responsible for a relatively slow progress in this research area and suggest most promising avenues for future research.

Outcomes of brood parasite-host interactions mediated by egg matching: common cuckoos Cuculus canorus versus Fringilla finches

BACKGROUND

Antagonistic species often interact via matching of phenotypes, and interactions between brood parasitic common cuckoos (Cuculus canorus) and their hosts constitute classic examples. The outcome of a parasitic event is often determined by the match between host and cuckoo eggs, giving rise to potentially strong associations between fitness and egg phenotype. Yet, empirical efforts aiming to document and understand the resulting evolutionary outcomes are in short supply.

METHODS/PRINCIPAL FINDINGS

We used avian color space models to analyze patterns of egg color variation within and between the cuckoo and two closely related hosts, the nomadic brambling (Fringilla montifringilla) and the site fidelic chaffinch (F. coelebs). We found that there is pronounced opportunity for disruptive selection on brambling egg coloration. The corresponding cuckoo host race has evolved egg colors that maximize fitness in both sympatric and allopatric brambling populations. By contrast, the chaffinch has a more bimodal egg color distribution consistent with the evolutionary direction predicted for the brambling. Whereas the brambling and its cuckoo host race show little geographical variation in their egg color distributions, the chaffinch's distribution becomes increasingly dissimilar to the brambling's distribution towards the core area of the brambling cuckoo host race.

CONCLUSION

High rates of brambling gene flow is likely to cool down coevolutionary hot spots by cancelling out the selection imposed by a patchily distributed cuckoo host race, thereby promoting a matching equilibrium. By contrast, the site fidelic chaffinch is more likely to respond to selection from adapting cuckoos, resulting in a markedly more bimodal egg color distribution. The geographic variation in the chaffinch's egg color distribution could reflect a historical gradient in parasitism pressure. Finally, marked cuckoo egg polymorphisms are unlikely to evolve in these systems unless the hosts evolve even more exquisite egg recognition capabilities than currently possessed.

Cuckoos versus hosts in insects and birds: adaptations, counter-adaptations and outcomes

Avian parents and social insect colonies are victimized by interspecific brood parasites-cheats that procure costly care for their dependent offspring by leaving them in another species' nursery. Birds and insects defend themselves from attack by brood parasites; their defences in turn select counter-strategies in the parasite, thus setting in motion antagonistic co-evolution between the two parties. Despite their considerable taxonomic disparity, here we show striking parallels in the way that co-evolution between brood parasites and their hosts proceeds in insects and birds. First, we identify five types of co-evolutionary arms race from the empirical literature, which are common to both systems. These are: (a) directional co-evolution of weaponry and armoury; (b) furtiveness in the parasite countered by strategies in the host to expose the parasite; (c) specialist parasites mimicking hosts who escape by diversifying their genetic signatures; (d) generalist parasites mimicking hosts who escape by favouring signatures that force specialization in the parasite; and (e) parasites using crypsis to evade recognition by hosts who then simplify their signatures to make the parasite more detectable. Arms races a and c are well characterized in the theoretical literature on co-evolution, but the other types have received little or no formal theoretical attention. Empirical work suggests that hosts are doomed to lose arms races b and e to the parasite, in the sense that parasites typically evade host defences and successfully parasitize the nest. Nevertheless hosts may win when the co-evolutionary trajectory follows arms race a, c or d. Next, we show that there are four common outcomes of the co-evolutionary arms race for hosts. These are: (1) successful resistance; (2) the evolution of defence portfolios (or multiple lines of resistance); (3) acceptance of the parasite; and (4) tolerance of the parasite. The particular outcome is not determined by the type of preceding arms race but depends more on whether hosts or parasites control the co-evolutionary trajectory: tolerance is an outcome that parasites inflict on hosts, whereas the other three outcomes are more dependent on properties intrinsic to the host species. Finally, our review highlights considerable interspecific variation in the complexity and depth of host defence portfolios. Whether this variation is adaptive or merely reflects evolutionary lag is unclear. We propose an adaptive explanation, which centres on the relative strength of two opposing processes: strategy-facilitation, in which one line of host defence promotes the evolution of another form of resistance, and strategy-blocking, in which one line of defence may relax selection on another so completely that it causes it to decay. We suggest that when strategy-facilitation outweighs strategy-blocking, hosts will possess complex defence portfolios and we identify selective conditions in which this is likely to be the case.