Interstitial Arabidopsis-Type Telomeric Repeats in Asteraceae

Abstract

Tandem repeats of telomeric-like motifs at intra-chromosomal regions, known as interstitial telomeric repeats (ITR), have drawn attention as potential markers of structural changes, which might convey information about evolutionary relationships if preserved through time. Building on our previous work that reported outstanding ITR polymorphisms in the genus Anacyclus, we undertook a survey across 132 Asteraceae species, focusing on the six most speciose subfamilies and considering all the ITR data published to date. The goal was to assess whether the presence, site number, and chromosomal location of ITRs convey any phylogenetic signal. We conducted fluorescent in situ hybridization (FISH) using an Arabidopsis-type telomeric sequence as a probe on karyotypes obtained from mitotic chromosomes. FISH signals of ITR sites were detected in species of subfamilies Asteroideae, Carduoideae, Cichorioideae, Gymnarhenoideae, and Mutisioideae, but not in Barnadesioideae. Although six small subfamilies have not yet been sampled, altogether, our results suggest that the dynamics of ITR formation in Asteraceae cannot accurately trace the complex karyological evolution that occurred since the early diversification of this family. Thus, ITRs do not convey a reliable signal at deep or shallow phylogenetic levels and cannot help to delimitate taxonomic categories, a conclusion that might also hold for other important families such as Fabaceae.

Keywords: Arabidopsis-type ITR; Asteraceae; Asterales; FISH; cytogenetic evolution; interstitial telomeric repeats.

Figure 1

Telomeric-like sequence (TTTAGGG)_n sites in Asteroideae species assessed by FISH analysis. Telomeric sites and interstitial telomeric repeat (ITR) sites are shown as red fluorescent signals and the chromosomes are counterstained with 4, 6-diamidino-2-phenylindole (DAPI) (blue colour). (A) Cota nigellifolia (2n = 18), showing 52 ITR sites. (B) Senecio vulgaris (2n = 40,) with two ITR sites (arrowhead) located in the same chromosome. Scale bars: 10 µm.

Figure 2

Telomeric-like sequence (TTTAGGG)_n sites in representative species of Carduoideae (A,B), Mutisioideae (C) and Gymnarhenoideae (D) tribes assessed by FISH analysis. Telomeric sites and interstitial telomeric repeat (ITR) sites are shown as red fluorescent signals and the chromosomes are counterstained with 4, 6-diamidino-2-phenylindole (DAPI) (blue colour). (A) Galactites tomentosa (2n = 20) showing 10 ITR sites. (B) Carlina hispanica (2n = 18 + 2B) with six ITR sites. (C) Nassauvia sprengelioides (2n = 44) with four ITR sites). (D) Gymnarrhena micrantha (2n = 20) showing ten ITR sites. Faint ITR signals are marked with arrowheads. Scale bars: 10 µm.

Figure 3

Telomeric-like sequence (TTTAGGG)_n sites in representative species of Cichorioideae (A–D) tribes assessed by FISH analysis. Telomeric sites and interstitial telomeric repeat (ITR) sites are shown as red fluorescent signals and the chromosomes are counterstained with 4, 6-diamidino-2-phenylindole (DAPI) (blue colour). (A) Stokesia laevis (2n = 14) with four sites. Two chromosomes were missed from the metaphase plate. (B) Leontodon longirostris (2n = 8) with ten ITR sites. (C) Hyoseris taurina (2n = 16) with 18 ITR sites. (D) Sonchus tenerrimus subsp. tenerrimus (2n = 14) showing two sites. Scale bars: 10 µm.

Figure 4

Differences in the signal intensity of ITR sites detected in Asteraceae chromosomes. (1): Anacyclus clavatus; (2): Leontodon tuberosus; (3–5): Hyoseris taurina.

Figure 5

Overview of the phylogenetic relationships among tribes of Asteraceae [27]. The proportion of sampled genera for each analyzed tribe is indicated. Tribes showing ITR sites are boxed.

Figure 6

Relation between number of ITR sites and haploid chromosome number (n) in Asteraceae.

Figure 7

Longitudinal distribution of ITR sites along the chromosome arm in Asteraceae. The frequency in each chromosomal region is indicated. The chromosome arm was divided into four major domains of unequal size, i.e. centromeric (c), proximal (p), interstitial-proximal (ip), and interstitial-terminal (it). (A). Data from the whole dataset. (B) Data excluding Anacyclus.