Karyotypic Uniformity in the Red-Winged Blackbird

Abstract

-Karyotypes of 52 Red-winged Blackbirds (Agelaius phoeniceus) from four widely separated populations were analyzed for possible regional differences in diploid numbers (2n) and in the sizes and shapes of the seven largest chromosomes. G-banded karyotypes for a subset of these individuals were compared to assess the banding homology between two of the populations. Results thus far indicate that Red-winged Blackbirds are karyotypically invariant across much of their geographic range and have an estimated 2n of 76. The recent discoveries of intraspecific chromosome polymorphisms in several avian species (Thorneycroft 1966, 1976; Shields 1973, 1982; Ansari and Kaul 1979a, b; Bass 1979; Kaul and Ansari 1979) have generated considerable new interest in avian cytogenetics. Therefore, the once common notion that birds are karyotypically conservative organisms (Ohno et al. 1964, Takagi and Sasaki 1974) needs to be reevaluated, especially in relation to recent speculation on the role that chromosomal polymorphisms may play in speciation processes (White 1978, but see Shields 1982). Intraspecific chromosomal variation may be more common in birds than is presently recognized. In order to detect such variation, one must sample the karyotypes of many individuals. Most avian karyotypes, however, have been determined from too few individuals to yield information about intraspecific variation. Consider a hypothetical example in which an avian species possesses a pair of chromosomes with two different morphologies because of a pericentric inversion. If the polymorphism exists in Hardy-Weinberg equilibrium with relative frequencies of p = 0.8 and q = 0.2 for the two morphs, then one may use a binomial distribution to calculate the probability of not detecting the lower(or higher-) frequency morph at given sample sizes. In this example, most of the individuals (p2 or 64%) are homozygous for the higher-frequency morph and thus yield no information about the presence or absence of any intraspecific polymorphism. The remaining 36% (2pq or 32% + q2 or 4%) exhibit the polymorphism in either the heteroor homozygous condition. If repeated samples of five individuals were karyotyped in this species, then the chromosomal polymorphism would be undetected in slightly over 10% of the samples (the exact probability is [0.64]5 = 0.1074). Because even five individuals are an unusually large sample in many avian cytoge etic studies, many of the 556 avian species so far karyotyped (de Boer 1983) may harbor undetected chromosomal polymorphisms. In addition, banded chromosomes can show differences that are not evident in standard karyotypes (Mateescu et al. 1974, Pollock and Fechheimer 1981, Shields 1983). In a recent review of avian cytogenetics, Shields (1982) contended that karyotypic variability has not been satisfactorily assessed in any avian species studied to date, regardless of the number of individuals karyotyped. He argued that detailed Cand G-banded karyotypes are seldom used, and in no instance have current techniques been used to karyotype several individuals from different populations within the geographic range of a species. We present here the results of an extensive karyological study of the mitotic chromosomes of the Red-winged Blackbird (Agelaius phoeniceus). Between 9 and 20 individual birds from each of four widely separated breeding populations were analyzed for differences in the sizes and shapes of the seven largest chromosomes (hereafter referred to as macrochromosomes) and also for differences in diploid chromosome numbers (2n). A subset of these karyotypes was G-banded for comparisons of banding homologies between two populations. The Red-winged Blackbird has considerable morphological size and shape variation across its North American range (Power 1970, James 1983), with 14 subspecies recognized by the American Ornithologists' Union (1957). The extent to which this phenotypic variation covaries with genetic variation (either chromosomal or electromorphic) is unknown, but such information is crucial to understanding evolutionary processes in this species, as well as other avian species. Although chromosomal morphology is just one spect of the total genetic constituency of any