Chromosoma (Berl) (1983) 88:249-255 CHROMOSOMA 9 Springer-Verlag 1983 C-banding and non-homologous associations II. The "parachute" Xyp sex bivalent and the behavior of heterochromatic segments in Epilachnapaenulata M.E. Drets, E. Corbella, F. Panzera, and G.A. Folle Division of Human Cytogenetics and Quantitative Microscopy, Instituto de Investigaciones Biol6gicas Clemente Estable, Avda. Italia 3318, Montevideo, Uruguay Abstract. A chromosome complement formed by 16 au- tosomes 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 chromo- some set carried large paracentric heterochromatic C-seg- ments representing about 15% of the haploid complement length. Heterochromatic segments associated progressively during early meiotic stages forming a large single chromo- center. 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 un- til early diakinesis. Thin remnant filaments were also ob- served 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 "can- opy" 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 para- centric C-segment of the X and Y chromosomes; the eu- chromatic 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 forma- tion 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 heterochro- matic corpuscles to opposite poles is described. These find- ings suggest that in E. paenulata the Xyp sex bivalent forma- tion 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. Introduction Nonhomologous chromosome association involving several or all chromosomes in a diploid cell seems to be a rather frequent process in insect meiosis. These associations are generally produced through heterochromatic segments lo- cated at the telomere region or at the paracentric area. An extreme case of a generalized end-to-end random associ- ation involving all bivalents was observed in Gryllus argen- tinus (Drets and Stoll 1974). In this cricket, manifold chro- mosome connections occurred through heterochromatic terminal segments which persisted until mid diakinesis. Condensed heterochromatic segments were found connect- ing all bivalents, a fact related to the presence of telomeric C-segments in mitotic chromosomes. In beetles heterochromatic segments usually undergo different degrees of ectopic pairing and chromocenter for- mation (Virkki 1951). Similar nonhomologous pairing and chromocenter fusion were first reported in plants by Heitz (1933) who noticed the tendency of heterochromatic seg- ments from different chromosomes to associate into large chromocenters. In the present paper, observations on the karyotype, size and distribution of constitutive C-segments, their be- havior during chromocenter formation and the organiza- tion of the Xy v "parachute" sex bivalent in Epilachna pae- nulata Germar are described. Material and methods Specimens of E. paenulata were collected during summer- time in suburban areas of Montevideo, Uruguay or in plan- tations of Cucurbitapepo and C. maxima usually parasitized by the insect. The animal was also successfully reared in the laboratory which allowed the study of specimens of known age the whole year round. The method used to rear E. paenulata consisted of introducing two or three females and one male into a flask containing fresh leaves of C. pepo or small pieces of the fruit (about 0.5 cm long) and a slightly humidified, sterile filter paper stacked at the bottom. Flasks were maintained at 20o22 ~ C and checked daily for eggs. Eggs were transferred to another flask as soon as they were laid to prevent the parents from eating them. Eclosion oc- curred after approximately 2 weeks, and larvae were fed with fresh leaves of C. pepo grown in a greenhouse. The complete larval cycle was about 5-6 weeks. Two-day-old adult male specimens were selected to obtain early meiotic stages. A high number of cells per slide was obtained by dissecting and pooling testes of 5-7 animals. Cell dispersion technique and saline solution have been reported previously (Drets and Stoll 1974), except that the trypsin step was skipped to avoid an excessive digestion and chromosome loss. Cells were fixed in methanol acetic acid (3:1) and