Evolution of satellite DNAs from the genus Palorus--experimental evidence for the "library" hypothesis

Abstract

Satellite DNA profiles have been characterized in the congeneric species Palorus ratzeburgii, Palorus subdepressus, Palorus genalis, and Palorus ficicola (Coleoptera, Insecta), each of which contains a single, AϩT-rich satellite DNA comprising a considerable portion of the genome (20%-40%). These satellites exhibit insignificant mutual sequence similarity. Using PCR assay, it has been shown that all four sequences are present in each of the tested Palorus species: one of them is amplified into a high copy number or a major satellite, while the three others are in the form of low-copy-number repeats estimated to make up approximately 0.05% of the genome. Each of the four satellites is interspecifically highly conserved concerning the sequence, monomer length, and tandem repeat organization. Major, as well as low-copy-number, satellites are colocalized in the regions of pericentromeric heterochromatin on all chromosomes of the complement. The low-copy-number satellites are dispersed between the large arrays of the major satellite over the whole heterochromatic block. Our results explain satellite DNA evolution, confirming the hypothesis that related species share a ''library'' of conserved satellite sequences, some of which could be amplified into a major satellite. Due to the evolutionary dynamics of satellite DNAs, the content of the ''library'' is variable; the elimination of some sequences parallels the creation of the new ones. Quantitative changes in satellite DNAs, induced by occasional amplification of satellite repeat from the ''library,'' could possibly occur in the course of the speciation process, thus forming a species-specific profile of satellite DNAs.

Long-term conservation vs high sequence divergence: the case of an extraordinarily old satellite DNA in bivalve mollusks

“…Their position within the variability profile of other monomers indicates a recent origin by amplification of a variant from a common pool, probably in the course or after speciation. This distribution is consistent with the satDNA library model (Fry and Salser, 1977;Meštrović et al, 1998Meštrović et al, , 2006a, but with monomer variants acting as independent amplification-contraction units. Their random amplification in the library (Pons et al, 2004) can explain the dominance of a homogeneous cluster of monomers in M. arenaria, the abundance of divergent variants lacking species specificity, or the combination of both features in V. decussata and V. rhomboides, respectively.…”

“…Source arrays (Figure 5c) and those formed by mutated monomers (Figure 5d) will presumably evolve independently because of sequence differences among diverged monomers. The resulting diverged repeats can be considered as elements of a satDNA library (Fry and Salser, 1977;Meštrović et al, 1998). An additional source of modified repeats can be in gradually degenerated homogeneous arrays predicted to form the final stage in the satDNA life cycle (Figure 5e; Nijman and Lenstra, 2001).…”

“…Large differences in satDNA sequences observed among related species are traditionally interpreted as a consequence of accumulated changes in separate lineages in the absence of selective pressure. The library model explains the origin of these differences by amplification-contraction events in a set of satDNAs shared by a group of organisms, a phenomenon that does not necessarily include rapid sequence alterations (Fry and Salser, 1977;Meštrović et al, 1998Meštrović et al, , 2006aUgarković and Plohl, 2002). In this way, one or a few satDNAs can become highly represented in a taxon, whereas others remain as low-copy number repeats, often not easily detected.…”

Satellite DNA in insects: a review

“…The copy number of satellite repeats could change mainly by unequal crossing-over events, although other processes, such as replication slippage, rolling-circle replication, conversion-like mechanisms and other unknown mechanisms, could also be involved (reviewed by Charlesworth et al, 1994). Mestrovic et al (1998) demonstrated experimentally, for the first time to our knowledge, some of the postulates of this 'library model'. In coleopteran genera Palorus and Pimelia, it appears that a common ancestor bore the majority of the major satDNA from the present species.…”

Sequence variability of the MspI satellite DNA family of the pinewood nematode Bursaphelenchus xylophilus at different geographic scales

“…Despite rapid changes in copy numbers, satDNAs are inherited in a linear fashion through generations, and their genomic composition is expected to follow species evolutionary history. Sequence rearrangements such as duplications, insertions, deletions, and inversions within monomers may play a dominant role in the evolution of some satellite sequences (Mravinac and Plohl, 2007), and rearranged monomer variants often follow the principles of the ''library model'', which is based on the differential amplification/contraction of satDNA subfamilies through species/ lineages diversification (Salser et al, 1976;Meštrović et al, 1998). In this model, related species share a ''library'' of conserved satDNA sequence variants, some of which could be amplified into a major satellite due to the evolutionary dynamics of satDNAs.…”