Painting analysis of meiotic metaphase I configurations of the germ line-limited chromosomes in Acricotopus.

Meiotic metaphase I configurations and pairing behavior of the germ line-limited chromosomes (= Ks) in the chironomid Acricotopus lucidus were analyzed by chromosome painting using specific probes of the three soma chromosomes (= Ss) and of their individual arms. The Ks are derived from the Ss and possess large S-homologous sections. Beside regular K and S bivalents, we also observed frequently K multivalents, e.g. trivalents, mainly quadrivalents, but also penta- and hexavalents, composed of the same K type in metaphases I. Chiasmata predominately occur within the S-homologous sections, probably ensuring a correct segregation and the transmission of a set of Ks to the next generation. Because K bivalents are almost exclusively autobivalents in A. lucidus formed by earlier sister chromatids, this multivalent formation with crossover also between homologous but non-identical Ks leads to genetic recombination within a K type. Rarely, quadrivalents composed of non-homologous Ks but derived from the same S were found. Therefore, these multivalents most probably resulted from crossover between homologous sections of morphologically different K types. This may result in new K types and might be important for the evolution of K type diversity in A. lucidus. In some cases, pairing-like associations between SIII and K4, which is derived from SIII, were observed in metaphases, indicating the possibility of crossover events and recombination between these chromosomes and so between the somatic and the germ-line restricted chromosome complements. Possible functions of additional copies of S sequences carried in the germ line are discussed.