Chromatin condensation dynamics during spermatogenesis and variation in detectability and spatial distribution of satellite DNAs outside constitutive heterochromatin in Tenebrio molitor.

Repetitive DNA sequences comprise a significant proportion of eukaryotic genomes and play an important role in genome structure, function, and evolution. The genome of the beetle Tenebrio molitor is particularly rich in satellite DNAs (satDNAs), which represent a prominent component of its repetitive DNA fraction. While dominantly found in long arrays confined to heterochromatin, satDNAs can also be found in short arrays in extra-heterochromatic regions. However, their cytogenetic detectability and spatial distribution can be influenced by chromatin condensation and epigenetic context. In this brief report, we investigated chromatin condensation dynamics during male germline development in T. molitor and examined how these dynamics affect the detection and localization of high- and low-copy satDNAs. DNA staining, immunodetection of the H3K9me3 epigenetic mark of constitutive heterochromatin, and fluorescence in situ hybridization (FISH) of satDNA sequences were used to examine the relationship between chromatin organization, the functional state of heterochromatin, and satDNAs positioning and detectability. Our results revealed that a large, central, and highly condensed chromosomal core is enriched in H3K9me3 and corresponds to constitutive heterochromatin. Major satDNA forms the bulk of this compartment and remains readily detectable regardless of chromatin condensation level. In contrast, the detectability of low-copy satDNAs decreased with increasing chromatin condensation. Additionally, numerous weak, punctate signals belonging to low-copy satDNAs were observed outside the DAPI-bright chromosomal mass, located in very loosely condensed chromatin, largely excluded from the heterochromatic chromosomal core. These findings demonstrate that chromatin condensation and constitutive heterochromatin organization strongly influence satDNA detectability and spatial distribution in T. molitor.