MOLECULAR GENETIC BASIS FOR THE FORMATION OF ECOLOGICAL RACES IN CUCKOOS (CUCULIFORMES, AVES)

N.N. Balatsky

Key words

Cuckoo, nest parasite species, host species, ecological race, egg, egg-shell coloration .

 

Abstract

 

Egg-shell coloration was studied in 370 eggs of nest parasite species of cuckoos (Cuculus canorus, C. saturatus, C. micropterus, C. poliocephalus, Hierococcyx fugax). In 281 nests (76%) background coloration and pattern of the egg-shell, as well as architectonics and pattern location on the egg-shell, are concordant in the parasite species and the host species. In the cases of polymorphic coloration of the egg-shell in a host species, egg-shell coloration in the parasite species mimics an average variant of the egg-shell in the host species. This concordance implies an analogous molecular-genetic basis for the phenomena. A new hypothesis is put forward of  regulative synthesis of the egg-shell coloration in the parasite species.

 

Introduction

 

The present work concerns the study of the diversity of ecological races of cuckoo species (Cuculus canorus, C. saturatus, C. micropterus, C. poliocephalus, Hierococcyx fugax) of Eurasia (Balatsky 1988, 1989, 1990, 1991a-c, 1992, 1993, 1994a-c, 1995, 1997a-b, 1998a-c, Balatsky et al. 1993a-b, 1994, 1995a-b, 1997, 1998, 1999a-b) through an analysis of the egg-shell coloration. Under an ecological race I imply a local population of a parasite species whose individuals are for many generations are reproductively connected with a certain host species in the nest of which the cuckoos lay their eggs of a similar coloration. In two control areas in West Siberia and North Primorye I have carried out a long term mo0nitoring of qualitative changes in the egg shell coloration in C. canorus parasiting in the nests of Blyth’s warbler Herbicola dumetorum and Thick-billed warbler Phragmaticola aeedon.

 

Material and methods

 

In nature and museum oological collections I examined nests of 64 bird species with 370 cuckoo eggs. The background coloration and elements of the pattern on the egg shell in pairs of a parasite species and host species was scored externally and against the light in an ovoscope.

 

Results

 

We reveal two coloration categories of the eggs of parasite species. In the first category (76% of eggs) the background coloration and drawing, and also the pattern architectonics and its location on the egg shell are adequate both on the surface and within the shell (Fig.). The first coloration category is found in all the cuckoos (C. canorus, C. saturatus, C. micropterus, C. poliocephalus, H. fugax). In the second category (24% of eggs) the background coloration and drawing, and also disposition of the pattern elements on the egg shell in parasite species and host species are not concordant. The second category was met with in C. canorus and C. saturatus.

 

Fig. The egg of C. canorus (left) and of host species Emberiza cioides (right)
under external illumination (A) and in ovoscope peek-a-boo (B).

 

Monitoring for qualitative changes in the egg shell coloration in C. canorus, parasiting in the nests of Herbicola dumetorum, has shown the following. With the drawing being stable (brown spots, black dots, marble ornament), in the eggs of Herbicola dumetorum the background coloration varies from greenish to pink. The eggs of an ecological race "Herbicola dumetorum" of the parasite species have an adequate drawing but the range of variation of the background is much narrower, from light-greenish to light-pinkish (Balatsky, 1988, 1998а). An analogous situation is found also in the case of the egg shell background coloration in an ecological race "Lanius cristatus" of C. micropterus (Balatsky, Nikolaev,1993a-b), and an ecological race "Communis communis" of C. canorus (Balatsky, 1998 c). hence, the egg shell coloration in a parasite species equals to an averaged variant of that in a host species.

Discussion

 

The second category appeared to contain cuckoo eggs both of specific ecological races but laid into the eggs of accessory or accidental hosts and those of deviating coloration. Concerning the second coloration category one can conclude that discordance of egg coloration in a pair of parasite species and host species is brought about ecological, ethological or genetic factors. In such cases many eggs of the parasite species as a rule are eliminating by the hosts while incubating. Few bird species are indifferent to the coloration of the eggs added (Saxicola torquata, Cyanoptila cyanomelana). The greatest number of the eggs of C. canorus of the second category is observed in Europe where ecological circumstances are on average less favorable for establishing of long-term connections between parasite and host species (Balatsky, 1995). A frequent change of host species leads to diffusion of the ecological races of a species and, as a consequence, to impoverishment of genetic diversity of populations of the latter. This is reflected in the egg shell coloration of the cuckoos: fragments of several coloration types mimic to different host species (deviating coloration) occur simultaneously.

The first category embraces the majority of eggs of all the parasite species, that characterizes the normal situation for nest parasitism. On the Russian territory three cuckoo species have one host species: C. micropterus – Lanius cristatus; C. poliocephalus – Horeites diphone; H. fugax – Larvivora cyane. Large reproductive populations of C. canorus are differentiated into 46 local ecological races (host species  – Anthus, Motacilla, Budytes, Emberiza, Sylvia, Communis, Lanius, Prunella, Locustella, Phragmaticola, Herbicola, Acrocephalus, Sollicitus, Leptopoecile, Phoenicurus, Erithacus, Calliope, Cyanosylvia, Fringilla, Acanthis), while those of C. saturatus into 5 local ecological races (Phylloscopus, Acanthopneuste, Reguloides). An adequate concordance of coloration elements of the cuckoo shells with those of the main host species implies a new concept of formation of the egg shell coloration in parasite species. A trivial selection of cuckoo eggs through their elimination of the nest hosts (in cases of non-concordant coloration) does not solve many problems, especially with respect to a precise concordance of the pattern elements on the egg shell surface and, that is more important, inside the shell. For host species this is of no value, while in their nests they often adopt cuckoo eggs with  deviations in coloration.

A new hypothesis is suggested of a regulative synthesis of the egg coloration in the host species. An adequate concordance of the coloration elements of the eggs of the parasite species with those in the host species imply an analogous molecular-genetic basis for this phenomenon. Looks like the host species during feeding of the cuckoo nestling exerts a directed influence on the genome of the young female of the parasite species. Mediators of such an unusual process may be some proteins expressed by the corresponding genes of the host species during breeding period, which are responsible for reproduction in general and the egg shell coloration in particular. Getting to a developing organism of a young cuckoo and acing no barrier of yet not fully established immune system the foreign host species proteins, gradually for a chain of generations, affect the corresponding genes of the parasite species. In this way ecological the races of the latter are being formed. It was noticed that changes first take place in the ground color and then formation of new pattern elements occur. This way the process of reduction of the former coloration and formation of a new one is going on.

 

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