Understanding the extensive hybridization in South American Nothofagus through karyotype analysis

Abstract

Extensive phylogeographic and population studies in Nothofagus revealed that hybridization, introgression and plastid capture are common phenomena that have occurred throughout the evolutionary history of the genus. Here, detailed karyotypes of ten South American Nothofagus spp. were constructed using chromosome fluorescent banding with the aim of (1) investigating if karyotype features were compatible with the formation of fertile interspecific hybrids, particularly when growing in sympatry and (2) identifying species-specific chromosomal markers to enable further studies of hybridization in Nothofagus. Similar karyotype formulas and heterochromatin patterns among species of the same clade (i.e. subgenus) revealed a low rate of chromosomal change. This finding reinforces the idea that hybrids between Nothofagus spp. can be fertile and that chromosome pairing in meioses could be successful. Genome conservation and extensive hybridization that resulted in plastid capture has been observed in other woody genera. Hybridization in tree species could be a survival strategy to enable the successful colonization of sites after disturbance and the introgression of genes from their congeners (adaptive introgression) may play an important role in adapting to climate change. Finally, N. antarctica has one more nucleolus organizing region (NOR) than its congeners that is easily identifiable and therefore could be used in future studies of hybrids.

An extraordinarily stable karyotype of the woody Populus species revealed by chromosome painting

“…We hypothesize that woody species are also slower than herbaceous plants to generate and fix rearranged chromosomes or altered chromosome numbers. Karyotyping of multiple species within the same genus has been conducted in several genera of woody plants, including Pinus (Hizume et al, 2002), Nothofagus (Acosta and Premoli, 2018) and Eucalyptus (Ribeiro et al, 2016). Intriguingly, all these studies demonstrated a highly similar karyotype among different species in the same genus.…”

Hybrid identification in Nothofagus subgenus using high resolution melting with ITS and trnL approach

“…Cycles of hybridization-introgression and chloroplast capture among extant and ancestral taxa result in concordant cpDNA phylogeographic patterns, whereas nuclear DNA (ITS) illustrates relationships among delimited species (Acosta & Premoli, 2010). In addition, karyotype conservation can contribute to explaining the existence of extensive “plastid capture” that has been observed in wood taxa (sensu: Acosta & Premoli, 2018).…”

“…The same phenomenon is found within New Zealand species, where the hybridization occurs among them; this has been recorded by Poole (1951) and Poole (1958). In addition, hybridizations have been recorded in cultivation out of natural habitat range for distinct Nothofagus species of different continental distribution but always among the same subgenus, that suggests a strong degree of intrinsic reproductive isolation among the different subgenus (Heenan & Smissen, 2013; Acosta & Premoli, 2018). For subgenus Nothofagus , the natural hybridization process which has been confirmed for the three evergreen species (Donoso & Atienza, 1983).…”

“…Therefore, phonological overlap between N . antarctica with N. pumilio at their lower elevational extremes and with N. dombeyi at mid elevations may prompt greater opportunities for the potential formation of hybrids (Acosta & Premoli, 2018). On the other hand, the authors have shown that the high conservation of karyotypes found in woody species may account for the presence of the fertile hybrids occurring naturally between species from the same section, clade linage.…”

What about Adaptive Introgression? The Genomic Revolution Is Shaping Scientific and Public Perception

“…Several studies recognise adaptive introgression as bene cial for species tness (Acosta and Premoli, 2018, Aeschbacher et al 2017, Hohmann and Koch, 2017, Horn et al 2018, Lehnert et al 2019, Ma et al 2019, Paun et al 2016, Semenov et al 2018. Possible reasons for the bene t of introgression are the potential of adaptive introgression for promoting individual survival (Hejase andLiu, 2016, Norris et al 2015), range expansion (Whitney et al 2015), species divergence (Morales et al 2017), evolutionary rescue (Quigley et al 2019) and adaptive radiation (Poelstra et al 2018, Zhang et al 2016.…”