Abstract
Bimodal karyotypes are characterized by the presence of two sets of chromosomes of contrasting size. Eleutherine bulbosa (2n = 12) presents a bimodal karyotype with a large chromosome pair, which has a pericentric inversion in permanent heterozygosity with suppressed recombination, and five pairs of three to four times smaller chromosomes. Aiming to understand whether high copy number sequence composition differs between both chromosome sets, we investigated the repetitive DNA fraction of E. bulbosa and compared it to the chromosomal organization of the related Eleutherine latifolia species, not containing the pericentric inversion. We also compared the repetitive sequence proportions between the heteromorphic large chromosomes of E. bulbosa and between E. bulbosa and E. latifolia to understand the influence of the chromosome inversion on the dynamics of repetitive sequences. The most abundant repetitive families of the genome showed a similar chromosomal distribution in both homologs of the large pair and in both species, apparently not influenced by the species-specific inversions. The repeat families Ebusat1 and Ebusat4 are localized interstitially only on the large chromosome pair, while Ebusat2 is located in the centromeric region of all chromosomes. The four most abundant retrotransposon lineages are accumulated in the large chromosome pair. Replication timing and distribution of epigenetic and transcriptional marks differ between large and small chromosomes. The differential distribution of retroelements appears to be related to the bimodal condition and is not influenced by the nonrecombining chromosome inversions in these species. Thus, the large and small chromosome subgenomes of the bimodal Eleutherine karyotype are differentially organized and probably evolved by repetitive sequences accumulation on the large chromosome set.
Highlights
Bimodal karyotypes are characterized by the presence of two sets of chromosomes of contrasting size
Bimodal Karyotype of E. bulbosa Is Characterized by a Differential Chromosome-Type Specific Repeat Distribution
The analysis revealed three major satellite DNA families, 11 transposable element families (LTR-retrotransposons and LINE), DNA transposons and ribosomal DNA sequences (Table 1)
Summary
Bimodal karyotypes are characterized by the presence of two sets of chromosomes of contrasting size. Gene content, the abundance of heterochromatic repetitive sequences, and the replication behavior differ between both chromosome sets (McQueen et al, 1998; Smith et al, 2000). A specific satDNA, found in Muscari comosum (Hyacinthaceae), is related to the heterochromatic bands of the large chromosomes, and it has been suggested to cause the increase of asymmetry of the karyotypes within this genus (de la Herrán et al, 2001). Independent of composition and origin, in both animal and plant bimodal species, it was suggested that the maintenance of these chromosome size differences could be related to the genome structure and function (Coullin et al, 2005; Vosa, 2005; Griffin et al, 2015)
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