Triplex configuration in the nick-free DNAs that constitute the chromosomal scaffolds in grasshopper spermatids

Abstract

After applying proper deoxyribonucleic acid (DNA) probes, fluorescence in situ hybridization (FISH) showed that the 8/9 centromeres-one per chromatid of the male haploid complement (X0) of Pyrgomorpha conica grasshopper-colocalized at the spermatid blunt end, where the spermatozoa flagellum inserts. A bundle of aligned 4',6-diamidino-2-phenylindole-positive chromatid scaffolds, which formed the central spermatid core, was observed after DNA breakage detection followed by FISH. Modular nature of scaffold DNA was occasionally evident. The technique also showed that in the early spermatid, the chromatid scaffolds lacked any DNA nick, whereas abundant breaks accumulated in the surrounding loops. Moreover, immunodetection showed that scaffold DNA participated in the formation of triplex DNA, while this configuration was absent from the loops. During spermatid maturation, triplex DNA disappeared from the scaffold in parallel with loop retraction, while protamines replace histones. Thus, the presence of triplex DNA in the chromatid scaffold correlates with the anchoring of expanded DNA loops to it. After loop retraction, the scaffolds of all chromatids coiled as a single unit in the spermatid head. This cooperative coiling produced enlargement and tilting of the distal telomeric signals, which were distributed along the spermatid head according to the length of each chromosome. We propose that specific DNA sequences dispersed throughout the whole chromatid fold forward and backward coaxially to chromatid length, forming individual scaffold modules whose linear assembly accounts for the minimum length of each individual chromatid. Finally, the core of the grasshopper male spermatid should be considered as a single chromosome in which the DNA scaffolds of the whole set of the nonhomologous chromosomes of the haploid complement are interconnected. This pattern of chromatin organization applies probably to other elongated spermatids.