Chromosomal mapping reveals a dynamic organization of the histone genes in aphids (Hemiptera: Aphididae)

Despite their involvement in different processes, histone genes have been analysed in few insects. In order to improve the knowledge about this important gene family, genes coding for histones have been analysed in the aphid Acyrthosiphon pisum showing that at the amino acid level, aphid histones are highly conserved. In particular, data from A. pisum confirm that H1 is the most variable of the five histones, whereas histones H3 and H4 are highly conserved with the H3 almost identical from insects to vertebrates. A. pisumhistone genes are organized in a quintet with the H1 gene followed by H2A and H2B genes that are adjacent and transcribed in same directions, in the opposite strand in respect to the H1 gene. At the 3’ end of the histone cluster, genes H3 and H4 constitute an oppositely transcribed pair. The span of the aphid histone genes (more than 7 kb) is greater than the average length of the histone cluster till now reported in insects (about 5 kb). Furthermore, spacers that separate the aphid histone genes vary in length. The histone genes have been mapped in A. pisum and successively in the aphids Myzus persicae and Rhopalosiphum padi showing that they are present in a single large cluster located in an interstitial position of autosomes 1, differently from what reported in the Russian wheat aphid Diuraphis noxia,where histone genes have been localized in a telomere of the two X chromosomes suggesting a dynamic organization of this multigene family in aphids

Analysis of insect holocentric chromosomes by atomic force microscopy

In order to go in depth into the analysis of holocentric chromosome structure, atomic force microscopy (AFM) was applied to metaphase plates of the aphid Megoura viciae. AFM showed that aphid chromatids adhere to one another without any prominent structure detectable between them and without any evidence of chromosomal constrictions. AFM thus provided new and reliable evidences at a nanomolecular level concerning the holocentric structure of aphid chromosomes, without any of the artefacts due to sample staining or coating that are usually associated with electron microscopy

Cytogenetic analysis on the holocentric chromosomes of the cabbage aphid Brevicoryne brassicae

Chromatin organization in the holocentric chromosomes of the aphid Brevicoryne brassicae has been investigated at a cytological level after C-banding, NOR, Giemsa, DAPI and CMA(3) staining. C-banding technique showed the presence of heterochromatic bands on both telomeres of the two X chromosomes, whereas only the longest pair of autosomes show distinct intercalary C-positive bands. Moreover, silver staining and fluorescent in situ hybridization (FISH) with a 28S rDNA probe localized rDNA genes on one telomere of each X chromosome; these are the only brightly fluorescent C-positive regions revealed after CMA(3) staining, whereas all other heterochromatic bands are DAPI positive

Holocentric chromosomes

Holocentric chromosomes possess multiple kinetochores along their length rather than the single centromere typical of other chromosomes. They have been described for the first time in cytogenetic experiments dating from 1935 and, since this first observation, the term holocentric chromosome has referred to chromosomes that: i. lack the primary constriction corresponding to centromere observed in monocentric chromosomes; ii. possess multiple kinetochores dispersed along the chromosomal axis so that microtubules bind to chromosomes along their entire length and move broadside to the pole from the metaphase plate. These chromosomes are also termed holokinetic, because, during cell division, chromatids move apart in parallel and do not form the classical V-shaped figures typical of monocentric chromosomes. Holocentric chromosomes evolved several times during both animal and plant evolution and are currently reported in about eight hundred diverse species, including plants, insects, arachnids and nematodes. As a consequence of their diffuse kinetochores, holocentric chromosomes may stabilize chromosomal fragments favouring karyotype rearrangements. However, holocentric chromosome may also present limitations to crossing over causing a restriction of the number of chiasma in bivalents and may cause a restructuring of meiotic divisions resulting in an inverted meiosis

Evolving aphids: one genome-one organism insects or holobionts?

Aphids have obligate mutualistic relationships with microorganisms that provide them with essential substances lacking in their diet, together with symbionts conferring them conditional adaptive advantages related, for instance, to the thermal tolerance and to the resistance to parasitoid wasps. The presence/absence of a secondary symbiont may have a relevant phenotypic effect so that aphid microbial symbionts constitute a sort of second genome with its own genetic inheritance. On the whole, genes important for aphid survival and reproduction are not uniquely present in the aphid nuclear and mitochondrial genomes, but also in the chromosomes of each symbiont. As a consequence, aphids should be viewed as holobionts with an extended genome (the hologenome) including the host and its symbiotic microbiome. In this connection, the true unit of selection in evolution must be considered the aphid holobiont, in place of the single host as individual separated from its symbiont

Karyotype rearrangements and telomere analysis in Myzus persicae (Hemiptera, Aphididae) strains collected on Lavandula sp. plants

Karyotype analysis of nine strains of the peach-potato aphid Myzus persicae (Sulzer, 1776), collected on Lavandula sp. plants, evidenced showed that five of them had a standard 2n = 12 karyotype, one possessed a fragmentation of the X chromosome occurring at the telomere opposite to the NOR-bearing one and three strains had a chromosome number 2n = 11 due to a non-reciprocal translocation of an autosome A3 onto an A1 chromosome. Interestingly, the terminal portion of the autosome A1 involved in the translocation was the same in all the three strains, as evidenced by FISH with the histone cluster as a probe. The study of telomeres in the M. persicae strain with the X fission evidenced that telomerase synthesised de novo telomeres at the breakpoints resulting in the stabilization of the chromosomal fragments. Lastly, despite the presence of a conserved telomerase, aphid genome is devoid of genes coding for shelterin, a complex of proteins involved in telomere functioning frequently reported as conserved in eukaryotes. The absence of this complex, also confirmed in the genome of other arthropods, suggests that the shift in the sequence of the telomeric repeats has been accompanied by other changes in the telomere components in arthropods in respect to other metazoans

Starting at the end: telomeres and telomerase in arthropods

Telomere composition and structure have been studied in several arthropods allowing us to better understand the evolution of such an important portion of the eukaryotic chromosomes. Genes coding for telomerase reverse transcriptase (TERT) have been sequenced and studied in few arthropod species only,where they resulted highly transcribed also in somatic tissues suggesting a different TERT regulation in respect to vertebrates. Contrary to the strict conservation of telomeres,subtelomeric regions were more polymorphic and heterogeneous in composition and frequently contained retrotransposable elements that strongly influenced subtelomere evolution

Cytogenetic and molecular characterization of the MBSAT1 satellite DNA in holokinetic chromosomes of the cabbage moth, Mamestra brassicae (Lepidoptera)

Digestion of Mamestra brassicae DNA with DraI produced a prominent fragment of approximately 200 by and a ladder of electrophoretic bands with molecular weights which are a multiple of 200 bp. Southern blotting revealed that this ladder is composed of DNA fragments that are multimers of the 200-by DraI band suggesting that DraI isolated a satellite that has been called Mamestra brassicae satellite DNA 1 (MBSAT1). MBSAT1 is the first satellite DNA isolated in Lepidoptera. In-situ DraI digestion of chromosome spreads, together with fluorescent in-situ hybridization, showed that MBSAT1 sequences are clustered in heterochromatin of the sex chromosomes, Z and W. MBSAT1 was 234 bp long with an AT content of 60.7%. The curvature-propensity plot suggested a curvature in the MBSAT1 structure

Distribution and molecular composition of heterochromatin in the holocentric chromosomes of the aphid Rhopalosiphum padi (Hemiptera: Aphididae)

In order to study the structure of holocentric chromosomes in aphids, the localization and the composition of Rhopalosiphum padi heterochromatin and rDNA genes have been evaluated at cytogenetic and molecular level. In particular, heterochromatin resulted located on all the chromosomes both in intercalary and telomeric positions. Moreover, enzymatic digestion of R. padi genome put in evidence a DraI satellite DNA which has been isolated, cloned and sequenced. FISH experiments showed that this satellite DNA clusters in an intercalary C-positive band on the two X chromosomes