Variability in Genetic Architecture of the Cryptic Species Complex of Acanthocyclops Vernalis (Copepoda). I. Evidence From Karyotypes, Genome Size, and Ribosomal Dna Sequences

Populations of the North American cryptic species complex of Acanthocylops vernalis (Fischer 1853) (Copepoda) possess unusually variable karyotypes and levels of reproductive isolation, but are difficult to discern morphologically. We established nine isofemale lines derived from four local and geographically isolated pond populations from Wisconsin and three isofemale lines from a lake in Ohio to explore the variability and relationships of chromosome numbers, genome sizes, and similarity of ribosomal DNA sequences. Five karyotypes (2n = 6, 7, 8, 9 and 10) were observed, although genome sizes were remarkably consistent. A 593 bp region of the 18S ribosomal gene was identical in four Wisconsin and three Ohio lines, but differed among five Wisconsin lines. This variability in 18S rDNA sequences among lines, which is uncharacteristic for well defined cyclopoid species, was used to construct a network of relationships among the isofemale lines. Mapping chromosome number onto this network revealed a cluster of lineages with variable chromosome numbers and a cluster with consistent chromosome number, although the overall pattern was more complex than this. Although 4-5 bivalents were observed in oocytes of the progeny from one isofemale line from Ohio, progeny of the latter had smaller genomes than any of the Wisconsin lines. All Wisconsin and Ohio lines lacked chromatin diminution, a trait previously attributed to some populations of A. vernalis by Standiford (1989, Genetika 79: 207-214) and Akifiev (1974, Priroda (USSR) 9: 49-54). Occurrence of chromosomal aberrations among disjunct populations may account for the unusual genetic variability of this species complex. Such differentiation also may be accelerated in fragmented habitats where genetic isolation seems to be recent and recurrent