A cytogenetic characterization, including heterochromatin content, and the analysis of the location of rDNA genes, was performed in Largus fasciatus Blanchard, 1843 and L. rufipennis Laporte, 1832. Mitotic and meiotic analyses revealed the same diploid chromosome number 2n = 12 + X0/XX (male/female). Heterochromatin content, very scarce in both species, revealed C-blocks at both ends of autosomes and X chromosome. The most remarkable cytological feature observed between both species was the different chromosome position of the NORs. This analysis allowed us to use the NORs as a cytological marker because two clusters of rDNA genes are located at one end of one pair of autosomes in L. fasciatus, whereas a single rDNA cluster is located at one terminal region of the X chromosome in L. rufipennis. Taking into account our results and previous data obtained in other heteropteran species, the conventional staining, chromosome bandings, and rDNA-FISH provide important chromosome markers for cytotaxonomy, karyotype evolution, and chromosome structure and organization studies.

The most important economic impact of Largus fasciatus and L. rufipennis is the feeding effect on potato, carrot, tobacco, and bean crops in Argentine. Largidae have a low diploid chromosome number, a sex chromosome system of X0/XX (male/female), except to one species, and a pair of m chromosomes, excluding the genus Largus. Cytogenetic and heterochromatin characterization, and the analysis of the location of rDNA genes between both species were performed. They possess a same diploid number 2n=12+X0/XX and very large chromosomes. The C-banding patterns showed discrete C-positive bands terminally located in all autosomes and the X chromosomes, which were observed in all stages of mitosis and meiosis. The main cytogenetic difference was detected in the location of the rDNA clusters. In L. fasciatus the hybridization signals were located at subterminal position in an autosomal bivalent, whereas in L. rufipennis a cluster of rDNA genes was located at one end of the X chromosome. As a result, the rDNA clusters are very useful tools for the study of the karyotype structure and chromosome evolution in groups with holokinetic chromosomes due to it can contribute to understand the karyotype evolution and taxonomic relationships among several taxa. On the other hand, the absence of m chromosomes and the presence of sex chromosome system X0 in species of Larginae could be considered as derived characters, which arose during karyotype evolution. We suggest that within this subfamily, the m chromosome pair could have lost its particular meiotic behaviour and became a regular autosomal pair, or fused to an autosomal pair, contributing thus to the increase in the chromosome size and, also, to the decrease in the chromosome number of the species. Therefore, chromosome studies are of great importance for understanding the evolutionary history of different insects groups because the data obtained, together with existing for other species, showed that different chromosome changes are involved in the evolution of the several species in the same family and even in Heteroptera.