Interstitial Telomeric-like Repeats (ITR) in Seed Plants as Assessed by Molecular Cytogenetic Techniques: A Review

Abstract

The discovery of telomeric repeats in interstitial regions of plant chromosomes (ITRs) through molecular cytogenetic techniques was achieved several decades ago. However, the information is scattered and has not been critically evaluated from an evolutionary perspective. Based on the analysis of currently available data, it is shown that ITRs are widespread in major evolutionary lineages sampled. However, their presence has been detected in only 45.6% of the analysed families, 26.7% of the sampled genera, and in 23.8% of the studied species. The number of ITR sites greatly varies among congeneric species and higher taxonomic units, and range from one to 72 signals. ITR signals mostly occurs as homozygous loci in most species, however, odd numbers of ITR sites reflecting a hemizygous state have been reported in both gymnosperm and angiosperm groups. Overall, the presence of ITRs appears to be poor predictors of phylogenetic and taxonomic relatedness at most hierarchical levels. The presence of ITRs and the number of sites are not significantly associated to the number of chromosomes. The longitudinal distribution of ITR sites along the chromosome arms indicates that more than half of the ITR presences are between proximal and terminal locations (49.5%), followed by proximal (29.0%) and centromeric (21.5%) arm regions. Intraspecific variation concerning ITR site number, chromosomal locations, and the differential presence on homologous chromosome pairs has been reported in unrelated groups, even at the population level. This hypervariability and dynamism may have likely been overlooked in many lineages due to the very low sample sizes often used in cytogenetic studies.

Keywords: chromosomal landmarks; in situ hybridisation; interstitial telomeric repeats; karyological evolution.

Figure 2

Occurrence of ITR sites in seed plants. The absence (grey colour) and presence (orange colour) in major lineages is illustrated. Unsampled groups are depicted in white. The overview of the phylogenetic relationships is based on [188] for Gymnosperms and [189] for Angiosperms.

Figure 1

Telomeric sequences (Arabidopsis-type repeats) in (A) Lysimachia minoricensis (Primulaceae) and (B) Anacyclus pyrethrum (Asteraceae). L. minoricensis lacks ITR repeats, whereas A. pyrethrum shows many ITR sites located at proximal and interstitial regions. Scale bars = 10 µm.

Figure 3

Taxonomic distribution of ITRs in the sampled lineages of seed plants. The number of recorded species is indicated for each group (orange colour). The circumscription of higher taxonomic lineages follows the hypothesis of the Angiosperm Phylogeny Website [187].

Figure 4

Longitudinal distribution of ITR sites along the chromosome arm in major lineages of gymnosperms and angiosperms. The percentage of occurrence in each chromosomal region is indicated. The chromosome arm was divided into three major domains of unequal size, i.e., centromeric (c), proximal (p), and interstitial (i).

Figure 5

Number of ITR sites reported for the major lineages of seed plants. The range and average number of ITR signals are represented.

Figure 6

Presence and absence of ITR sites in seed plants according to the haploid chromosome number (n) of the analysed species.