A deep dive into the ancestral chromosome number and genome size of flowering plants

Abstract

Chromosome number and genome variation in flowering plants have stimulated growing speculation about the ancestral chromosome number of angiosperms, but estimates so far remain equivocal. We used a probabilistic approach to model haploid chromosome number (n) changes along a phylogeny embracing more than 10 000 taxa, to reconstruct the ancestral chromosome number of the common ancestor of extant angiosperms and the most recent common ancestor for single angiosperm families. Independently, we carried out an analysis of 1C genome size evolution, including over 5000 taxa. Our analyses revealed an ancestral haploid chromosome number for angiosperms of n = 7, a diploid status, and an ancestral 1C of 1.73 pg. For 160 families, inferred ancestral n are provided for the first time. Both descending dysploidy and polyploidy played crucial roles in chromosome number evolution. While descending dysploidy is equally distributed early and late across the phylogeny, polyploidy is detected mainly towards the tips. Similarly, 1C genome size also increases (or decreases) significantly in late-branching lineages. Therefore, no evidence exists of a clear link between ancestral chromosome numbers and ancient polyploidization events, suggesting that further insights are needed to elucidate the organization of genome packaging into chromosomes.

Karyotype and genome size variation in white-flowered Eranthis sect. Shibateranthis (Ranunculaceae)

“…In this case, we are sure that this mutation is just a polymorphic variant. Carta et al (2020) estimated x = 7 as the most likely ancestral basic chromosome number in Ranunculaceae. However, we hypothesize that, in Siberian species, E. sibirica and E. tanhoensis, the basic chromosome number evolutionarily reduced from x = 8 to x = 7 and not vice versa.…”

Early diversification and karyotype evolution of flowering plants

“…However, the rapid rise and early diversi cation of angiosperms has remained "an abominable mystery" since the time of Darwin 21 and estimates of the ancestral chromosome base number for angiosperms vary from x=5 to x=9, although x=7 is mostly likely 22 . Increasing molecular phylogenetic analyses of extant angiosperms have identi ed four major lineages: Amborellales, Nymphaeales and Austrobaileyales as the ANA grade and a core angiosperm lineage (Mesangiospermae) that includes the vast majority of the remaining owering plant diversity [9][10][11][12][13][14][15][16][17][18][19] .…”

Chromosome-levelde novogenome assembly ofTelopea speciosissima(New South Wales waratah) using long-reads, linked-reads and Hi-C

“…The Macadamia genome (2n = 28) has six more chromosomes than the Telopea genome (2n = 22), but the two species have similar estimated genome sizes -896 Mb [14] compared to 874 Mb. It is thought that the ancestral Proteaceae had a chromosome number of x = 7 [72][73][74][75], although the occurrence of paleo-polyploidy in family has been debated [76]. Overall, synteny analyses reveal an abundance of interchromosomal rearrangements between the Telopea and Macadamia genomes, reflecting the long time since their divergence (73-83 Ma [77]).…”