C-banding and non-homologous associations. II: The «parachute» Xyp sex bivalent and the behavior of heterochromatic segments in Epilachna paenulata

A chromosome complement formed by 16 autosomes and an Xyp sex chromosome system was found in Epilachna paenulata Germar (Coleoptera: Coccinellidae). All autosomes were metacentric except pair 1 which was submetacentric. The X and the Y chromosomes were also submetacentric but the Y was minute. The whole chromosome set carried large paracentric heterochromatic C-segments representing about 15% of the haploid complement length. Heterochromatic segments associated progressively during early meiotic stages forming a large single chromocenter. After C-banding, chromocenters revealed an inner networklike filamentous structure. Starlike chromosome configurations resulted from the attachment of bivalents to the chromocenters. These associations were followed until early diakinesis. Thin remnant filaments were also observed connecting metaphase I chromosomes. Evidence is presented that, in this species, the Xyp bivalent resulted from an end-to-end association of the long arms of the sex chromosomes. The “parachute” Xyp bivalent appeared to be composed of three distinct segments: two intensely heterochromatic C-banded corpuscles formed the “canopy” and a V-shaped euchromatic filament connecting them represented the “parachutist” component. The triple constitution of the sex bivalent was interpreted as follows: each heterochromatic corpuscle corresponded to the paracentric C-segment of the X and Y chromosomes; the euchromatic filament represented mainly the long arm of the X chromosome terminally associated with the long arm of the Y chromosome. The complete sequence of the formation of the Xyp bivalent starting from nonassociated sex chromosomes in early meiotic stages, and progressing through pairing of heterochromatic segments, coiling of the euchromatic filament, and movement of the heterochromatic corpuscles to opposite poles is described. These findings suggest that in E. paenulata the Xyp sex bivalent formation is different than in other coleopteran species and that constitutive heterochromatic segments play an important role not only in chromosome associations but also in the Xyp formation.