Hybridization‐facilitated genome merger and repeated chromosome fusion after 8 million years

Abstract

The small genus Ricotia (nine species, Brassicaceae) is confined to the eastern Mediterranean. By comparative chromosome painting and a dated multi-gene chloroplast phylogeny, we reconstructed the origin and subsequent evolution of Ricotia. The ancestral Ricotia genome originated through hybridization between two older genomes with n = 7 and n = 8 chromosomes, respectively, on the Turkish mainland during the Early Miocene (c. 17.8 million years ago, Ma). Since then, the allotetraploid (n = 15) genome has been altered by two independent descending dysploidies (DD) to n = 14 in Ricotia aucheri and the Tenuifolia clade (2 spp.). By the Late Miocene (c. 10 Ma), the latter clade started to evolve in the most diverse Ricotia core clade (6 spp.), the process preceded by a DD event to n = 13. It is noteworthy that this dysploidy was mediated by a unique chromosomal rearrangement, merging together the same two chromosomes as were merged during the origin of a fusion chromosome within the paternal n = 7 genome c. 20 Ma. This shows that within a time period of c. 8 Myr genome evolution can repeat itself and that structurally very similar chromosomes may originate repeatedly from the same ancestral chromosomes by different pathways (end-to-end translocation versus nested chromosome insertion).

Diversification of Biscutella ser. Biscutella (Brassicaceae) followed post-Miocene geologic and climatic changes in the Mediterranean basin

“…Biscutella and Megadenia in the early Miocene (ca 18.75 Mya) is fully congruent with recent estimation ofMandáková et al (2018), who placed it about 24.75-17.9 Mya (with crown at 21.67 Mya).…”

“…Biscutella, with the chromosome basic number x = 6 having appeared early in the evolution of the series, very likely following a dysploidy event (the remaining sections and series are x = 8, 9). This pattern might parallel the genome evolution shown by Mandáková et al (2018) Hsü et al 1973Hsü et al , 1978Bocquet et al, 1978;Krijgsman et al, 1999). The Iberian and African plates remained connected about 0.6 million years through a temporal bridge along the Betic-Rifan Arc (cf.…”

“…Biscutella. The available information concerning long-term evolutionary events in Brassicaceae dates the divergence time of Biscutelleae in about 27.4-25.4 Million years ago (Mya) (Huang et al, 2015), 20−17.5 Mya (Hohmann et al, 2015), 27.4-17.9 Mya (Mandáková et al, 2018), or 21.0-14.0 Mya (Huang et al, 2019), when the tribe diverged from other cruciferous lineages.…”

Born in the Mediterranean: Comprehensive Taxonomic Revision of Biscutella ser. Biscutella (Brassicaceae) Based on Morphological and Phylogenetic Data

“…Biscutella with the chromosome basic number x 5 6 being the probable result of dysploidy in the early evolution of the series (the remaining sections and series are x 5 8, 9) (see Alonso et al, 2020). Similar patterns of genome evolution have recently been reported by Mandáková et al (2018) in the related Ricotia, a genus in which descendant dysploidy played an important role in cladogenesis.…”

The chromosome-level genome sequence and karyotypic evolution of Megadenia pygmaea (Brassicaceae)

“…Comparative genomics and chromosome painting revealed that the ancestral karyotype of the Lineage I, the Ancestral Crucifer Karyotype (ACK), was comprised of eight chromosomes (n = 8) and 22 genomic blocks (GBs) (Lysak et al, 2016). The inferred ancestral karyotype of the Lineage II, the 'Proto-Calepineae Karyotype' (PCK, n = 7; Mandáková et al, 2018;Mandáková & Lysak, 2008), was found to be derived from the ancestral PCK (ancPCK, n = 8) through descending dysploidy, namely a reduction in chromosome number (Geiser et al, n.d.;Mandáková et al, 2018).…”