An analysis of population genetic differentiation and genotype–phenotype association across the hybrid zone of carrion and hooded crows using microsatellites and MC1R

The all black carrion crow ( Corvus corone corone ) and the grey and black hooded crow ( Corvus   corone cornix ) meet in a narrow hybrid zone across Europe. To evaluate the degree of genetic differentiation over the hybrid zone, we genotyped crows from the centre and edges of the zone, and from allopatric populations in northern (Scotland–Denmark–Sweden) and southern Europe (western–central northern Italy), at 18 microsatellites and at a plumage candidate gene, the MC1R gene. Allopatric and edge populations were significantly differentiated on microsatellites, and populations were isolated by distance over the hybrid zone in Italy. Single-locus analyses showed that one locus, CmeH9, differentiated populations on different sides of the zone at the same time as showing only weak separation of populations on the same side of the zone. Within the hybrid zone there was no differentiation of phenotypes at CmeH9 or at the set of microsatellites, no excess of heterozygotes among hybrids and low levels of linkage disequilibrium between markers. We did not detect any association between phenotypes and nucleotide variation at MC1R , and the two most common haplotypes occurred in very similar frequencies in carrion and hooded crows. That we found a similar degree of genetic differentiation between allopatric and edge populations irrespectively of their location in relation to the hybrid zone, no differentiation between phenotypes within the hybrid zone, and neither heterozygote excess nor consistent linkage disequilibrium in the hybrid zone, is striking considering that carrion and hooded crows are phenotypically distinct and sometimes recognised as separate species.     

Genetic variation of carrion and hooded crows and their hybrids based on RAPD-PCR data

RAPD-PCR analysis of genetic variability of carrion crows, hooded crows and their phenotypic hybrids from the zones of overlap of their areals and hybridization of parental forms in Siberia has been carried out. According to RAPD variability the following parameters were estimated for both species and their hybrids: genetical distances (DN), polymorphism (P95), mean expected heterozygosity (H(e)), gene fixation coefficient (Fst) and the portion of interpopulation differentiation (Gst). It was shown that the rate of genetic variability in carrion crows and hooded crows is less than in the hybrids. It turned out that the hybrids were genetically closer to each other (DN = 0.295) than to both parental species (DN = 0.441 and 0.397 with C. corone and C. cornix, correspondingly). The studied individuals of C. cornix show the minimal level of heterozygosity and polymorphism (H(e) = 0.12 and P95 = = 27.8%) in comparison with C. corone and the crows of hybrid zone (H(e) = 0.18 and 0.20; P95 = 42.6% and 50.4%, correspondingly). Right's indexes of inbreeding (Fst = 0.285) and population spliting (Gst = 0.352) indicate low genetic variability in all the analysed birds. Nevertheless UPGMA and NJ dendrogrammes differentiate the individuals from the areals of carrion crows, hooded crows and from hybrid zone to different clusters. Our data correlate with molecular-genetic investigations of Corvidae carried out with other methods, and do not contradict the conception of carrion crows and hooded crows as semispecies in the context of the conception of overspecies.     

Habitat preferences and positive assortative mating in an avian hybrid zone

The relatively narrow hybrid zone between hooded and carrion crow is characterised by a high proportion of breeding hybrids, no known intrinsic postzygotic costs, and most likely, high gene flow between populations. It is therefore of interest to identify factors preventing a breakdown of the zone. In this study, we investigate the possibility that habitat choice could create assortative mating in the German–Danish part of the zone. In a GIS analysis, we used two approaches to investigate the role of habitat selection in the maintenance of this zone. We tested if there are 1) any correlations between habitat and the shape of the hybrid zone, and 2) any differences in habitat preferences between territorial individuals of the crow phenotypes. The most extensive habitat, non‐irrigated arable land (Habitat 1), which covers almost 50% of the area in the zone was preferred by all three phenotypes (carrion, hooded and hybrid crows), but showed no change in frequency across the zone. Pastures (Habitat 2) and agricultural areas mixed with natural vegetation (Habitat 4) correlated with the shape of the zone, but only H4 differed in preference between the crow phenotypes. H4 was preferred by hybrids and hooded crows, but not by carrion crows. This habitat is present only in 17% of the hooded crow territories, and thus, it is not likely to have a strong influence on the maintenance of the hybrid zone. There was evidence of assortative mating in pure phenotypes, whereas hybrids showed no consistent mating pattern. Because our GIS analyses suggest that species specific habitat preferences are not involved in shaping the assortativeness, we discuss other possible mechanisms.     

Hybrid zone maintenance by non-adaptive mate choice

In a spatial simulation we show that formation of a stable, narrow hybrid zone requires no selection, only that individuals can recognize their own population type since this is a prerequisite for assortative mating. The European crow occurs in two species/subspecies that meet in a long hybrid zone, the black carrion crow Corvus corone and the grey and black hooded crow C. cornix. In a previous study (Brodin A, Haas F (2006) Anim Behav 72:139) we mimicked sexual imprinting in nestling crows with artificial neural networks, using a learning process that simulates proximate retina perception. The networks were trained on 700 images to recognize either carrion, hooded or hybrid phenotypes as their own. After training the networks were exposed to 300 new, unfamiliar pictures of crows. The networks that had been trained on pure subspecies then showed strong preference for their own type. Networks trained on hybrid crows showed weaker preference for their own type. Assuming that these preferences will determine mate choice preferences we have combined them with empirical data on dispersal of young crows to investigate what kind of geographical distribution pattern this would create. We then assume that the two subspecies meet and hybridize along a straight border. In only 60 generations a stable, narrow hybrid zone that resembles the real hybrid zone was formed. This zone remained stable over time. With minor adjustments of parameters the results would not only fit the width but also the shapes of the clines in the field. An erratum to this article can be found at     

Extra‐pair paternity as a strategy to reduce the costs of heterospecific reproduction? Insights from the crow hybrid zone

Within hybrid zones of socially monogamous species, the number of mating opportunities with a conspecific can be limited. As a consequence, individuals may mate with a heterospecific (social) partner despite possible fitness costs to their hybrid offspring. Extra‐pair copulations with a conspecific may thus arise as a possible post hoc strategy to reduce the costs of hybridization. We here assessed the rate of extra‐pair paternity in the hybrid zone between all‐black carrion crows (Corvus (corone) corone) and grey hooded crows (C. (c.) cornix) and tested whether extra‐pair paternity (EPP) was more likely in broods where parents differed in plumage colour. The proportion of broods with at least one extra‐pair offspring and the proportion of extra‐pair offspring were low overall (6.98% and 2.90%, respectively) with no evidence of hybrid broods having higher EPP rates than purebred nests.     

Aprocta matronensis in crows (Corvus corone corone) from northern Italy.

Aprocta matronensis (Nematoda, Spirurida) has been found in the orbital cavities of carrion crows. Hooded crows and crows from the hybrid crow zone in the studied areas were not infected. The parasite population showed the typical morphology described for this species. This is the first record of A. matronensis in crows in Italy and in Corvus corone corone. Some hypotheses about the distribution of the parasites in crow populations are discussed.     

The restriction analysis of nDNA from the corvids (Passeriformes,aves)

• 1.1. The nDNA of carrion crow Corvus corone L., hooded crow C. cornix L., their hybrids, as well as magpie Pica pica L., were digested by the tetranucleotide recognizing restriction enzymes Sau3a, AluI, BspRI and then analysed using electrophoresis with microdensitometry.
• 2.2. The distribution patterns of restriction DNA fragments proved to be nuclease- and taxon specific.
• 3.3. The observed families of repeated sequences are characterized by different length (from 30 bp to 23 tbp), number of copies in genome (approximately 10³ and 10⁶) and supposedly different types of organization and evolutionary age.
• 4.4. The total DNA amount identified in the form of discrete fragments is 16 and 19–21% for magpie and crows, respectively.
• 5.5. The DNA restriction patterns of hybrid forms do not differ from the parental species.

     

Gene‐flow across the European crow hybrid zone – a spatial simulation

In hybrid zones populations that are otherwise allopatric meet and produce hybrids. One of the most well‐known hybrid zones occurs throughout much of Europe between the hooded and carrion crows. Even though these species (or subspecies) of crows look very different, genetic differentiation is weak, and introgression seems to be common. In a spatial simulation that was based on empirically estimated values, we investigated how introgressing alleles that would confer some fitness advantage would flow across the zone. The advantage was assumed to be some unknown factor that enhanced survival for carriers relative to non‐carriers. We varied the yearly survival advantage between 0 to 10% and recorded how this would affect zone shape and position. In the simulation we cycled ‘yearly’ events such as reproduction, mortality and juvenile dispersal. We started the simulation by equipping all individuals of one crow type outside the zone with a homozygotic set of the beneficial allele. At all levels of advantage the allele first rapidly became fixed in the donator crow type, then transgressed into the receiving type more slowly and finally became fixed in all individuals. The time until fixation varied from around 176 yr for a 10% advantage to around 20 000 yr for 0% advantage. An exciting discovery was that the position of the zone would move during the introgression but then stop. The reason is that the beneficial allele would be common in the donating type but not in the receiving type during the introgression event. At large the crow hybrid zone appears to have been stable for a long time but there have been small recent northward movements of carrion crows in both Scotland and Denmark. We suggest that introgression by itself is a factor that should be considered as an explanation for why hybrid zones move temporarily.     

The exclusion of avian predators from aggregations of nesting lapwings (Vanellus vanellus)

Territorial lapwings in Aberdeenshire, Scotland, largely prevented carrion crows, (Corvus corone), their main egg predators, from approaching close to nest-sites and from entering the area within an aggregation of 11 nests. Predation rates of artifical nests were significantly lower when placed within 10 m of active lapwing nests than when>200 m from nest-sites, indicating that the presence of lapwings afforded protection from crow predation. Further artificial nest experiments showed that the amount of protection declined linearly with increasing distance from the nest-site, and some degree of protection occurred to at least 30–50 m from it. The size of this protected zone was apparently related to the number of lapwing nests present, and the survival time of eggs in artificial nests reflected the lapwings' nesting density. The defended zones around each lapwing nest overlapped appreciably in dense or large nesting aggregations, leading to the observed protection by generally excluding crows, from the nesting area.     

Mesopredators constrain a top predator: competitive release of ravens after culling crows

Although predator control programmes rarely consider complex competitive interactions among predators, it is becoming clear that removal of larger ‘superior’ competitors often releases the ‘inferior’ ones and can precipitate trophic cascades. In contrast, our study indicates that culling hooded crows Corvus cornix appears to release a larger competitor, the common raven Corvus corax. Ravens ranged more widely, and the predation of artificial nests was significantly faster (although total predation was similar), after the removal of crows. Our study provides evidence of a novel reversal of competitive release where a larger species was freed from constraints imposed on its distribution and behaviour by a smaller species, and emphasizes the importance of considering community and ecosystem effects of predator manipulations when undertaken for conservation or game management.     

THE MAINTENANCE OF A CLINE IN THE MARINE SNAIL LITTORINA SAXATILIS: THE ROLE OF HOME SITE ADVANTAGE AND HYBRID FITNESS

Steep clinal transitions in one or several inherited characters between genetically distinct populations are usually referred to as hybrid zones. Essentially two different mechanisms may maintain steep genetic clines. Either selection acts against hybrids that are unfit over the entire zone due to their mixed genetic origin (endogenous selection), or hybrids and parental types attain different fitness values in different parts of the cline (exogenous selection). Survival rate estimates of hybrids and parental forms in different regions of the cline may be used to distinguish between these models to assess how the cline is maintained. We used reciprocal transplants to test the relative survival rates of two parental ecotypes and their hybrids over microscale hybrid zones in the direct-developing marine snail Littorina saxatilis (Olivi) on the rocky shores of Galicia, Spain. One of the parental forms occupies upper and the other lower shores, and the hybrids are found at various proportions (1–38%) along with both parental forms in a midshore zone a few meters wide. The survival rate over one month was 39-52% of the native ecotype on upper shores, but only 2-8% for the lower-shore ecotype. In contrast, only 4-8% of the upper-shore ecotype but 53% of large (> 6 mm) and 8% of small (3-6 mm) native lower-shore ecotype survived in the lower shores. In the midshores, both the two parental ecotypes and the hybrids survived about equally well. Thus there is a considerable advantage for the native ecotypes in the upper and lower shores, while in the hybrid zone none of the morphs, hybrids included, are favored. This indicates that the dimorphism of L. saxatilis is maintained by steep cross-shore selection gradients, thus supporting the selection-gradient model of hybrid zones. We performed field and laboratory experiments that suggest physical factors and predation as important selective agents. Earlier studies indicate assortative mating between the two ecotypes in the midshore. This is unexpected in a hybrid zone maintained by selection gradients, and it seems as if the reproductive barrier compresses the hybrid zone considerably.     

Leaf physiology reflects environmental differences and cytoplasmic background in Ipomopsis (Polemoniaceae) hybrids

Natural hybridization can produce individuals that vary widely in fitness, depending upon the performance of particular genotypes in a given environment. In a hybrid zone with habitat heterogeneity, differences in physiological responses to abiotic conditions could influence the fitness and spatial distribution of hybrids and parental species. This study compared gas exchange physiology of Ipomopsis aggregata, I. tenuituba, and their natural hybrids in situ and assessed whether physiological differences were consistent with their native environmental conditions. We also produced reciprocal F2s in a greenhouse study to test for cytonuclear effects on water-use efficiency (WUE). The relative performance of natural hybrids and parentals was consistent with their native habitats: I. aggregata at the coolest, wettest locations had the lowest WUE, while hybrids from the most xeric sites had the highest WUE. In hybrids, the mechanism by which both natural and experimental hybrids achieved this high WUE depended on cytotype: those with I. tenuituba cytoplasm had reduced transpiration, while those with I. aggregata cytoplasm had an increased photosynthetic rate, consistent with patterns in the cytoplasmic parent. The high WUE in hybrids may contribute to their high survival in the dry center of the natural hybrid zone, consistent with environment-dependent models of hybrid zone dynamics.     

Cryptic and extensive hybridization between ancient lineages of American crows

Most species and therefore most hybrid zones have historically been defined using phenotypic characters. However, both speciation and hybridization can occur with negligible morphological differentiation. Recently developed genomic tools provide the means to better understand cryptic speciation and hybridization. The Northwestern Crow (Corvus caurinus) and American Crow (Corvus brachyrhynchos) are continuously distributed sister taxa that lack reliable traditional characters for identification. In this first population genomic study of Northwestern and American crows, we use genomic SNPs (nuDNA) and mtDNA to investigate the degree of genetic differentiation between these crows and the extent to which they may hybridize. Our results indicate that American and Northwestern crows have distinct evolutionary histories, supported by two nuDNA ancestry clusters and two 1.1%‐divergent mtDNA clades dating to the late Pleistocene, when glacial advances may have isolated crow populations in separate refugia. We document extensive hybridization, with geographic overlap of mtDNA clades and admixture of nuDNA across >900 km of western Washington and western British Columbia. This broad hybrid zone consists of late‐generation hybrids and backcrosses, but not recent (e.g., F1) hybrids. Nuclear DNA and mtDNA clines had concordant widths and were both centred in southwestern British Columbia, farther north than previously postulated. Overall, our results suggest a history of reticulate evolution in American and Northwestern crows, perhaps due to recurring neutral expansion(s) from Pleistocene glacial refugia followed by lineage fusion(s). However, we do not rule out a contributing role for more recent potential drivers of hybridization, such as expansion into human‐modified habitats.     

Bounded hybrid superiority in an avian hybrid zone: effects of mate, diet, and habitat choice

There has been considerable debate in the study of hybrid zones as to whether hybrids may be superior to parental types within the area of contact (bounded hybrid superiority). In birds, naturally occurring hybridization is relatively common, and hybridization within this group always involves mate choice. If hybrids are superior, females choosing heterospecific mates should be expected to show higher fitness under the conditions prevalent in the hybrid zone. Hybrid superiority under these circumstances would reduce reinforcement and thereby help to maintain the hybrid zone. To examine this issue, we studied reproductive performances of hybrids and parental species of gulls (Larus occidentalis and Larus glaucescens) at two colonies within a linear hybrid zone along the west coast of the United States. This hybrid zone contains predominantly gulls of intermediate phenotype. Previous studies indicated that hybrids were superior to one or both parental types, but provided no data on possible mechanisms that underlie this hybrid superiority. Using a hybrid index designed specifically for these species, we identified to phenotype more than 300 individuals associated with nests, including both individual males and females within 73 pairs in the central portion of the hybrid zone and 74 pairs in the northern portion of the hybrid zone. There was little evidence of assortative mating, and what little there was resulted solely because of pairings within intergrades. In the central hybrid zone, females paired with hybrid males produced larger clutches and hatched and fledged more chicks compared with females paired to western gull males. This was a result of heavy predation on eggs in sand habitat, where male western gulls established territories. In contrast, many hybrid males established territories in vegetated cover that was less vulnerable to predation. In the northern part of the hybrid zone, clutch size did not differ among pair categories, however, there were differences in hatching and fledging success, with females paired to hybrid males showing better success compared to females paired to glaucous-winged gull males. Hybrids showed better hatching and fledging success in the north because hybrids are more like western gulls than glaucous-winged gulls in foraging behavior, taking a higher percentage of fish in their diet, which enhances chick growth and survival. This is believed to be the first documentation of bounded hybrid superiority that delineates the mechanisms that underlie hybrid superiority.     

Morphological Variation in Wild Marmosets (Callithrix penicillata and C. geoffroyi) and Their Hybrids

Evolutionary theory and observation predict wider phenotypic variation in hybrids than parental species. Emergent phenotypic novelty in hybrids may in turn drive new adaptations or speciation by breaking parental phenotypic constraints. Primate hybridization is often documented through genetic evidence, but knowledge about the primate hybrid phenotype remains limited due to a small number of available studies on hybrid primate morphology. Here, we examine pelage and morphometric variation in two Brazilian marmoset species (Callithrix penicillata and C. geoffroyi) and their hybrids. Hybrids were sampled in an anthropogenic hybrid zone in the municipality of Viçosa, Minas Gerais state, Brazil. We analyzed hybrid facial and body pelage color variation, and compared 13 morphometric measures between hybrids and parental species. Five different hybrid facial morphotypes were observed, varying from intermediate to parental-like. Hybrid facial morphotypes were biased towards C. penicillata, suggesting that the pelage of this species may be dominant to that of C. geoffroyi in this context, and indicating that mate preference, and therefore gene flow/introgression, may be biased towards C. penicillata within the hybrid zone. Hybrid morphometric features were on average intermediate to parental species traits, but transgressive hybrids were also observed, suggesting that morphometric variation for the studied traits is consistent with Rieseberg’s complementary allele model. Finally, we observed a decoupling of facial patterning and size/shape in hybrids, relative to parent phenotypes, suggesting that an important factor driving phenotypic novelty within the Viçosa marmoset hybrid zone might be the loosening of evolutionary constraints on phenotypic trait integration.     

On the phylogenetic relationship of Corvinae birds (Aves, Corvidae) from data of partial sequencing of cytochrome b gene mitochondrial DNA

To establish phylogenetic relationships within the corvine birds at the interspecific and intergeneric levels, the sequence of the mitochondrial DNA cytochrome b gene was analyzed. The NJ, UPGMA, and MP trees showed similar clustering. Relationships between the jungle crow, on the one hand, and the rook and Australian raven, on the other hand, were closer than between the jungle crow and the hooded and carrion crows. Mitochondrial genome of Australian raven displayed the closest similarity to the ancestral genome of the genus Corvus. Populations inhabiting the eastern part of the carrion crow C. corone orientations area were statistically significantly subdivided into three lineages. These data also confirmed the hypothesis on the location of the carrion crow ancestral lineage in the southeastern part of the area. In general, the transition and transversion substitution levels, their relationships, and distribution over codon positions were similar to that already reported for birds. Synonymous transitions in the third codon position were the prevailing substitution type. Using standard calibration scales, the time of divergence between species and genera within the corvine family was estimated to be 3.1-4 and 3.8-8.8 Myr, respectively. The divergence time between the examined corvine birds and birds of paradise constituted from 8 to 10 Myr.     

Identification of 20 polymorphic microsatellite loci in European crow (Corvus corone) from existing passerine loci

The European crow (Corvus corone) occurs in two subspecies (or species) with distinct plumage coloration: the black carrion crow (C. c. corone) and the grey and black hooded crow (C. c. cornix). We tested 42 passerine microsatellite loci for amplification in the European crow and identified 20 loci that were both polymorphic and easy to score. In 50 individuals sampled in the Danish part of the species’ pan‐European hybrid zone, the number of alleles ranged between two and 21. One locus deviated from Hardy–Weinberg equilibrium and had a high estimated null allele frequency. These 20 loci were highly successful in amplifying polymorphic products also in other crow populations and in another Corvidae species, the rook (Corvus frugilegus).     

ESTIMATING THE MATING BEHAVIOR OF A PAIR OF HYBRIDIZING HELICONIUS SPECIES IN THE WILD

Premating isolation between incipient species is rarely studied in nature, even though mating tests in captivity may give an inaccurate picture of natural hybridization. We studied premating barriers between the warningly colored butterflies Heliconius erato and H. himera (Lepidoptera) in a narrow contact zone in Ecuador, where hybrids are found at low frequency. Eggs obtained from wild-mated females, supplemented with eggs and young larvae collected from the wild, were reared to adulthood. Adult color patterns of these progeny were then used to infer how their parents must have mated. Likelihood was used to estimate both the frequencies of potential parental genotypes from adult phenotypes collected in the wild, and the degree of assortative mating from the inferred parents. The frequencies of parental genotypes varied across the hybrid zone, but our statistical method allowed estimates of hybrid deficit and assortative mating to be integrated across all sites sampled. The best estimate of the frequency of F₁ and backcross hybrid adults in the center of the hybrid zone was 10%, with support limits (7.1%, 13.0%; support limits are asymptotically equivalent to 95% confidence limits). Mating was highly assortative: in the center of the hybrid zone the cross-mating probability between H. erato and H. himera was only 5% (0.3%, 21.4%). Wild hybrids themselves mated with both pure forms, and the probabilities that they mated in any direction were not significantly lower than those among conspecifics. These results are consistent with earlier laboratory studies on mate choice, and suggest that selection against hybrids must be strong to prevent formation of a hybrid swarm. Unfortunately, the wide support limits on mating behavior precluded a measure of the strength of selection from these data alone. Our statistical approach provides a useful general method for estimating mate choice in the wild.     

Interspecific Hybridization as a Tool to Understand Vocal Divergence: The Example of Crowing in Quail (Genus Coturnix)

Understanding the mechanisms that lead organisms to be separated into distinct species remains a challenge in evolutionary biology. Interspecific hybridization, which results from incomplete reproductive isolation, is a useful tool to investigate such mechanisms. In birds, interspecific hybridization is relatively frequent, despite the fact that closed species exhibit morphological and behavioural differences. Evolution of behaviour is difficult to investigate on a large timescale since it does not ‘fossilize’. Here I propose that calls of hybrid non-songbirds that develop without the influence of learning may help in understanding the gradual process that leads to vocal divergence during speciation. I recorded crows produced by the European quail (Coturnix c. coturnix), the domestic Japanese quail (Coturnix c. japonica) and their hybrids (F1, F2 and backcrosses). Most crowing patterns were intermediate to those of the parental species; some were similar to one or the other parental species, or not present in either parental species. I also observed vocal changes in hybrid crows during the breeding season and from one year to the other. This vocal variability resembles those observed during the ontogeny of the crow in quails. It is likely that similar mechanisms involved in vocal changes during ontogeny might have driven vocal divergence in the species of Palearctic quails. I suggest that hybrid crows might have resembled those produced by intermediary forms of quails during speciation.     

Karyological study of some Corvine birds (Corvidae, aves)

Karyotypes were studied in the hooded and carrion crows, their naturally occurred hybrids, the jungle crow, the azure-winged magpie (2n = 80 in all aforementioned birds), and the magpie (2n = 82). Corvine birds of Primorskii Krai were karyotyped for the first time. In addition to the similarity in the diploid chromosome sets, corvine birds were shown to have a similar structure of karyotype: in all studied birds, 14 macrochromosomes (Mchs) classified into three groups according to their size were detected. By karyotype structure, birds belonging to the same genus are similar. Some intergeneric differences are due to a change in the position of centromeres of the largest and sex chromosomes. Karyotypes of interspecific hybrids of crows are remarkable for the presence of heteromorphic (t/st) chromosome pair 2 in some individuals, which apparently does not affect their fecundity. Using differential C-banding, the sex chromosome W in female magpies was identified. In addition, heteromorphism was detected in C-bands of homologs of Mch pair 4 in the hooded crow. In the jungle crow, the azure-winged magpie, and the magpie, bright QH-bands and numerous G-bands were detected on Mchs and on some microchromosomes only. Active Ag-NOR-bands were detected on one macrochromosome pair in the magpie. In all, the karyotype structure of corvine birds is comparable to the basic structural scheme of the karyotype in the order Passeriformes, which confirms the concept of conservatism of the avian karyotype.