Future perspectives
While cross-ploidy hybridisation is likely more common than once thought, particularly in plants, there is still much uncertainty in our understanding of the phenomenon. Key priorities should be to broaden the taxonomic scope of study to understand the frequency of cross-ploidy hybridisation across the Tree of Life and to reveal potential factors that may promote or prevent it, and to employ new genomic sequencing and analytical approaches to investigate the genomic basis of this phenomenon.
In terms of establishing the frequency of cross-ploidy hybridisation, there is currently a dearth of information on animal examples, even though polyploid incidence can be high in some groups (e.g. insects, decapods, fish, and amphibians, Otto & Whitton, 2000). Further, while we found many angiosperm examples, nearly half (17 out of 42) were derived from the large families Asteraceae and Orchidaceae. A broader scope will determine whether there is a phylogenetic signal to the phenomenon, and which attributes, including ecological to genetic factors, facilitate cross-ploidy hybridisation and introgression.
In terms of studying the genomics of cross-ploidy hybridisation, this will allow us to more accurately understand the population dynamics of cross-ploidy hybrid zones (Zohren et al., 2016), as well as precisely determine parental genomic contributions to cross-ploidy hybrids and hybrid species (Bertioli et al., 2016). The latter point is particularly important, as hybrids may be introgressed at only a few loci in the genome. Detecting these few loci requires a high contiguity polyploid genome assembly, preferably with phase information, and new and emerging sequencing methods such as long-read sequencing are beginning to address these problems (Zhang et al., 2019). In addition, sequencing of diploid relatives, and the application of more advanced approaches for separating the two subgenomes, such as those based on characteristic profile of different repeat content and transposable elements from each parental progenitor (e.g. Cerca et al., 2022), will be instrumental for understanding which subgenomes introgress.
Further genomic sequencing, aided by experimental work, will also provide more detailed insights into the genetic mechanisms that allow chromosomes to pair in newly formed polyploid hybrids (Morgan et al., 2020), which is important in establishment and persistence of hybrids. For example, in wheat (Triticum aestivum )—an allohexaploid composed of 3 related genomes—a single locus, Ph1 , enforces strict homologous bivalent pairing. Suppression of Ph1 allows meiotic pairing of homoeologous chromosomes, facilitating recombination and introgression (Li et al., 2017). Whether similar mechanisms occur in other polyploid systems remains largely uncharacterised.
Given the extensive ploidy variation throughout plants and animals, and the high degree of hybridisation detected in these groups, cross-ploidy hybridisation may be more important in plant and animal evolution than is currently recognised.
Data Accessibility and Benefit-Sharing : No new data are included.
Funding : This work was developed from the doctoral dissertation of M.R.B. published in 2020, which was supported by a scholarship from the Biotechnology and Biological Sciences Research Council (grant number BB/M010996/1). A.D.T. was supported by the Natural Environment Research Council (grant number NE/L011336/1). The Royal Botanic Garden Edinburgh acknowledges funding from the Scottish Government’s Rural and Environment Science and Analytical Services Division.
Conflict of interest disclosure : No conflict to declare.
Acknowledgements: We thank Simon Martin, Duncan Cameron and Anthony Hall for useful discussions.
Author contributions: All authors contributed to the writing of the manuscript.