Causes and consequences of variation in nestling immune function

Defence against pathogens is a vital need of all living organisms that has led to the evolution of complex immune mechanisms. However, although immunocompetence the ability to resist pathogens and control infection has in recent decades become a focus for research in evolutionary ecology, the variation in immune function observed in natural populations is relatively little understood. This thesis examines sources of this variation (environmental, genetic and maternal effects) during the nestling stage and its fitness consequences in wild populations of passerines: the blue tit (Cyanistes caeruleus) and the collared flycatcher (Ficedula albicollis). A developing organism may face a dilemma as to whether to allocate limited resources to growth or to immune defences. The optimal level of investment in immunity is shaped inherently by specific requirements of the environment. If the probability of contracting infection is low, maintaining high growth rates even at the expense of immune function may be advantageous for nestlings, as body mass is usually a good predictor of post-fledging survival. In experiments with blue tits and haematophagous hen fleas (Ceratophyllus gallinae) using two methods, methionine supplementation (to manipulate nestlings resource allocation to cellular immune function) and food supplementation (to increase resource availability), I confirmed that there is a trade-off between growth and immunity and that the abundance of ectoparasites is an environmental factor affecting allocation of resources to immune function. A cross-fostering experiment also revealed that environmental heterogeneity in terms of abundance of ectoparasites may contribute to maintaining additive genetic variation in immunity and other traits. Animal model analysis of extensive data collected from the population of collared flycatchers on Gotland (Sweden) allowed examination of the narrow-sense heritability of PHA-response the most commonly used index of cellular immunocompetence in avian studies. PHA-response is not heritable in this population, but is subject to a non-heritable origin (presumably maternal) effect. However, experimental manipulation of yolk androgen levels indicates that the mechanism of the maternal effect in PHA-response is not in ovo deposition of androgens. The relationship between PHA-response and recruitment was studied for over 1300 collared flycatcher nestlings. Multivariate selection analysis shows that it is body mass, not PHA-response, that is under direct selection. PHA-response appears to be related to recruitment because of its positive relationship with body mass. These results imply that either PHA-response fails to capture the immune mechanisms that are relevant for defence against pathogens encountered by fledglings or that the selection pressure from parasites is not as strong as commonly assumed.Lintujen pesäpoikasten immuunivasteen vaihtelun syyt ja seuraukset Kaikille eliöille on elintärkeää, että ne pystyvät puolustautumaan taudinaiheuttajia vastaan. Tämä tarve on johtanut monimutkaisten immuunipuolustusjärjestelmien evoluutioon. Vaikka immunokompetenssia eli kykyä vastustaa taudinaiheuttajia ja säädellä infektioita on viime vuosikymmeninä tutkittu paljon evoluutioekologian näkökulmasta, imuunipuolustuksen vaihtelua populaatioissa ei vielä ymmärretä riittävän hyvin. Tässä väitöskirjassa tutkittiin sinitiaisen (Cyanistes caeruleus) ja sepelsiepon (Ficedula albicollis) pesäpoikasvaiheessa ilmenevän immuunikompetenssin vaihtelun syitä (ympäristötekijät, perimä ja maternaalivaikutukset) sekä vaihtelun vaikutusta yksilöiden kelpoisuuteen. Yksilönkehityksen aikana linnunpoikasten on ratkaistava ongelma: sijoittaako rajalliset voimavarat kasvuun vai immuunipuolustukseen; molempien optimaaliseen kehittymiseen ei riitä resursseja. Pohjimmiltaan immuunipuolustuksen voimakkuuteen vaikuttavat ympäristön erityispiirteet. Jos infektoituminen on epätodennäköistä, pesäpoikasten kannattaa kasvaa mahdollisimman rivakasti immuunipuolustuksen kustannuksella, sillä poikasen suuri koko parantaa pesästä lähdön jälkeistä selviytymistä. Työssä tutkittiin kokeellisesti verta imevien kanakirppujen (Ceratophyllus gallinae) vaikutusta sinitiaisen poikasten kasvuun ja immuunivasteen kehittymiseen. Kokeessa poikasille annettiin joko metioniinia, joka vaikuttaa immuunivasteeseen käytettävien resurssien määrään tai lisäravintoa. Koe osoitti, että poikasten kasvu ja immuunivasteen kehittyminen kilpailivat rajallisista resursseista: nopea kasvu aiheutti alentuneen immuunivasteen ja päinvastoin. Kun kasvuympäristössä oli paljon ulkoloisia, poikaset sijoittivat voimavaroja enemmän immuunivasteen kehittymiseen. Eräissä kokeissa poikasia siirrettiin pesistä toisiin. Kokeet osoittivat, että ulkoloisten määrällä mitattu ympäristövaihtelu voi pitää yllä immuunivasteen ja eräiden muiden ominaisuuksien perinnöllistä vaihtelua. Väitöskirjassa tutkittiin myös ns. PHA-vasteen periytyvyyttä analysoimalla suurta Gotlannista kerättyä sepelsieppoaineistoa ns. Animal Model analyysimenetelmällä. PHA-vaste, jolla mitataan immunokompetenssia, ei määräydy Gotlannin sepelsieppopopulaatiossa perinnöllisesti vaan mitä todennäköisimmin maternaalisesti. Munankeltuaisen androgeenitason manipulointi osoitti, että emolinnut eivät eritä kehittyviin muniin androgeeniä. PHA-vasteen kehittyminen ei siten aiheudu emon munaan siirtämistä androgeeneistä. PHA-vasteen vaikutusta sepelsiepon poikasten rekrytoitumiseen Gotlannin pesivään populaatioon tutkittiin analysoimalla 1300 poikasen ominaisuuksia. Analyysi osoitti ruumiinpainon, ei PHA-vasteen, olevan luonnon valinnan kohteena. PHA-vaste on näennäisesti yhteydessä rekrytoitumistodennäköisyyteen, koska hyväkuntoisilla poikasilla on parempi PHA-vaste. Tulosten perusteella PHA-vaste ei joko ilmennä olennaisia pesäpoikasten kohtaamien patogeenien herättämän immuunivasteen mekanismeja tai sitten loisten aiheuttama valintapaine ei ole niin voimakas kuin aiemmin on luultu

Costs and Benefits of Experimentally Induced Changes in the Allocation of Growth versus Immune Function under Differential Exposure to Ectoparasites

Peer reviewe

Low but contrasting neutral genetic differentiation shaped by winter temperature in European great tits

Gene flow is usually thought to reduce genetic divergence and impede local adaptation by homogenising gene pools between populations. However, evidence for local adaptation and phenotypic differentiation in highly mobile species, experiencing high levels of gene flow, is emerging. Assessing population genetic structure at different spatial scales is thus a crucial step towards understanding mechanisms underlying intraspecific differentiation and diversification. Here, we studied the population genetic structure of a highly mobile species – the great tit Parus major – at different spatial scales. We analysed 884 individuals from 30 sites across Europe including 10 close-by sites (< 50 km), using 22 microsatellite markers. Overall we found a low but significant genetic differentiation among sites (FST = 0.008). Genetic differentiation was higher, and genetic diversity lower, in south-western Europe. These regional differences were statistically best explained by winter temperature. Overall, our results suggest that great tits form a single patchy metapopulation across Europe, in which genetic differentiation is independent of geographical distance and gene flow may be regulated by environmental factors via movements related to winter severity. This might have important implications for the evolutionary trajectories of sub-populations, especially in the context of climate change, and calls for future investigations of local differences in costs and benefits of philopatry at large scales

Analysis of the effect of ectoparasite load manipulation (parasites), methionine supplementation (methionine) and interaction between these treatments on growth of blue tit nestlings during the period of methionine supplementation (days 3–6) and immediately after termination of supplementation (days 6–9, day 9).

<p>Statistics of a linear mixed model with brood nested in parasite treatment fitted as a random effect; variance component, its standard error and percentage of variance explained by brood effect nested in the parasite treatment is shown. N equals the number of nestlings measured.</p

Survival of blue tit nestlings after the start of methionine supplementation (day 3 post-hatching).

<p>During the supplementation period (between day 3 and 6) methionine-treated nestlings (M, filled symbols, N = 209) had higher mortality than control nestlings (C, open symbols, N = 212).</p

Analysis of the effect of ectoparasite load manipulation (parasites) methionine supplementation (methionine) and interaction between these treatments on final body size of blue tit nestlings.

<p>Statistics of a linear mixed model with brood nested in parasite treatment fitted as a random effect; variance component, its standard error and percentage of variance explained by brood effect nested in the parasite treatment is shown. N equals the number of nestlings measured.</p

Mass gain of control (C, open symbols) and methionine-supplemented (M, filled symbols) blue tit nestlings in deparasitized and parasitized nests; least square means + SE.

<p>(A) During the supplementation period (between day 3 and 6) methionine treated nestlings had suppressed growth, but in deparasitized nests not significantly so. (B) Immediately after supplementation ended (between day 6 and 9) methionine treated nestlings had higher mass gain than control nestlings in parasitized nests, but not in deparasitized nests. Sample sizes are indicated on the graphs. Statistics in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0010814#pone-0010814-t002" target="_blank">Table 2</a>.</p

Physiological traits of control (C, open symbols) and methionine-supplemented (M, filled symbols) blue tit nestlings reared in deparasitized and parasitized nests; least square means ± SE.

<p>Sample sizes (number of nestlings) are indicated on the graphs.</p

Analysis of the effect of ectoparasite load manipulation (parasites), methionine supplementation (methionine) and interaction between these treatments on physiological traits of blue tit nestlings.

<p>Statistics of a linear mixed model with brood nested in parasite treatment fitted as a random effect; variance component, its standard error and percentage of variance explained by brood effect nested in the parasite treatment is shown. N equals the number of nestlings assayed.</p