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GENETIC DIFFERENCES BETWEEN CUCKOOS PARASITIZING BROODS OF TWO
SYMPATRIC SPECIES OF WARBLERS FROM THE ACROCEPHALUS GENUS
Joanna Sudyka, 1st year PhD studies co-financed by the European Union under the European Social Fund
Population Ecology Group, Institute of Environmental Sciences, Jagiellonian University
The strategy of common cuckoo (Cuculus canorus) involves parasitizing broods of other species. Previous studies allowed to identify host-specific genets
(races). Each of them aims in imitating background colour and spot pattern of host eggs. However these races appear to be limited to female lineage with
cross mating males keeping the cuckoos as one genetic species. The differences are reflected in variation on the level of mtDNA, suggesting that only females
are host-specific and genes responsible for this situation are located on chromosome W(1). Other work reveals nuclear microsatellites differentiation indicating
the possibility that genes responsible for evolution and maintenance of races could be found on autosomal chromosomes(2). The preliminary data obtained by
the Group from the genetic studies of cuckoo eggs found in reed warbler and great reed warbler nests suggest that even within the same genet classified on
the basis of egg morphology considerable genetic differences occur.
The main question: Is there a significant genetic differentiation between cuckoos
belonging to the same phenotypic genet parasitizing on different host species?
The common cuckoo
What is genet?
Females belonging to each genet selectively lay their eggs in specific host’s nest. Genets, also called races, are distinguished on the basis of
egg characteristics and are usually mimetic to host’s eggs. It is believed that races are maintained only by the females’ behaviour or
preferences and inherited through female sex chromosome W. Yet the view that the variation is present only in maternally inherited
mitochondrial DNA(1) was challenged by the work revealing diversification of nuclear microsatellite loci(2). This would mean that also males
are host-specific and display genets.
Kruszewicz A., Ptaki Polski, tom.1, Warszawa 2010
The comparative studies will be performed on cuckoos
parasitizing on great reed warbler (Acrocephalus
arundinaceus) and reed warbler (Acrocephalus scirpaceus).
Those are believed to belong to one genet on the basis of
egg phenotypic features (Sylvia egg type). But how is it
maintained on genetic level? Genetic differences between
cuckoos parasitizing on each species were observed even
though their egg morphology was the same.
Great reed warbler (left ) is larger (18-20 cm, 35-40 g) than reed warbler (13-14 cm, 12-14 g)
Eggs of different genets(3)
Which one belongs to the cuckoo?
The materials will be gathered in the area of fish ponds located in the
Milicz valley during three subsequent breeding seasons. Cuckoo eggs
from both host species shall be collected and examined (also with
spectrophotometer). Twelve cuckoo eggs have already been collected
last season. Egg white and embryo will be extracted for further analysis.
The cuckoo chicks and adult cuckoos will be blood sampled.
To assess variability 411 basepair portions coming from hypervariable
control region of mtDNA and 10 autosomal markers previously
applied(1,2) will be employed. Additionally the eggshells resistance to
controlled puncture will be studied in order to assess more differences
occurring between cuckoos specializing in both host species.
Feeding is costly for the host
If the differences between cuckoos parasitizing on both host species will be confirmed for large sample it would denote that genets can no longer be
distinguished solely on the basis of eggs’ look. It would show that even morphologically the same races include genetic diversity, which may also reveal recent
host switches in studied population. If the variation will be observed also on nuclear microsatellite level it would suggest that mate choice is not random, but
biased towards specific host preference for both: males and females.
Gibbs, H.L., M. Sorenson, K. Marchetti, M. Brooke, N.B. Davies, and H. Nakamura. 2000. Genetic evidence for female host-specific races of the common cuckoo. Nature 407: 183-186.
Fossøy F., Antonov A., Moksnes A., Røskaft E., Vikan J.R., Møller A.P., Shykoff J.A., Stokke B.G., Genetic differentiation among sympatric cuckoo host races: males matter, Proc. R. Soc. B published online 10 November 2010.
Stoddard M.C., Stevens M., Avian vision and the evolution of egg color mimicry in the common cuckoo, Evolution, July 2011, 2004-2013.