ARE LYCOPODS WITH HIGH CHROMOSOME NUMBERS ANCIENT POLYPLOIDS?

It has been established that the number of isozymes (different forms of an enzyme encoded by different gene loci) is highly conserved in diploid angiosperms and gymnosperms. In contrast, allopolyploid angiosperms display an increase in isozyme number due to the addition of divergent genomes. Lycopods (Microphyllophyta) are an ancient lineage of vascular plants having very high chromosome numbers. It has been maintained that lycopods acquired these high chromosome numbers through repeated episodes of polyploidy. Despite high chromosome numbers, however, lycopod species having the lowest chromosome numbers within genera possess the number of isozymes typical of diploid seed plants for all enzymes examined except triosephosphate isomerase. There is, therefore, no genetic evidence from enzyme electrophoresis for polyploidy in these plants. These results are comparable to findings for other homosporous pteridophytes including the ferns (Pteridophyta) and horsetails (Arthrophyta). Alternative hypotheses for widespread genetic diploidy in homosporous pteridophytes are 1) repeated cycles of allopolyploidy followed by gene silencing; 2) repeated cycles of autopolyploidy, which would result in duplicated, but not divergent genes for isozymes; 3) initiation of these lineages with relatively high chromosome numbers.     

ELECTROPHORESIS IS MODIFYING OUR CONCEPTS OF EVOLUTION IN HOMOSPOROUS PTERIDOPHYTES

Analyses of electrophoretically detectable enzyme variants in homosporous pteridophytes are facilitating the development of new insights into their genetics and evolution. The number of isozymes per enzyme indicates that homosporous pteridophytes are genetic diploids, in spite of the fact that they have high chromosome numbers. High levels of heterozygosity and genetic variability in sporophytic populations indicate that many diploid species are outcrossing with inbreeding representing a derived character state. Because the congeneric homosporous pteridophyte species analyzed to date have low genetic identities, allozymic characters are also proving to be useful as genomic markers for elucidating patterns of reticulate evolution. The accumulated data suggest that the genetic system of homosporous pteridophytes differs fundamentally from that of seed plants. The present genomic constitution of extant taxa may be the result of repeated cycles of allopolyploidy followed by gene silencing and extinction of progenitor taxa. Alternatively, the original homosporous pteridophytes may have had high chromosome numbers. Although current species probably evolved recently, their phylogenetic roots may be difficult to trace because even closely related pteridophytes are genetically distant and extinction has obliterated the ancestral intermediates between lineages. These hypotheses can and should be tested using a combination of molecular, phylogenetic, and population biology methods.     

Assessment of gene copy number in the homosporous fernsCeratopteris thalictroides andC. richardii (Parkeriaceae) by restriction fragment length polymorphisms

Homosporous ferns are generally considered polyploid due to high chromosome numbers, but genetically diploid since the expression of isozymes is generally controlled by a single locus. Gene silencing over evolutionary time is one means by which this apparent contradiction can be explained. A prediction of this hypothesis is that silenced gene sequences still reside in the genomes of homosporous ferns. We examined the genomes ofCeratopteris richardii andC. thalictroides for sequences which are similar to expressed gene sequences. Genomic DNA blots hybridized withC. richardii cDNA clones showed that the majority of these clones detected multiple fragments, suggesting that most gene-like sequences are duplicated inCeratopteris. Hybridization signal intensity often varied between fragments of the same size between accessions, sometimes dramatically, which indicates that not all sequences are equivalent, and may represent the products of silenced genes. Observed reciprocal differences in intensity could be due to reciprocally silenced genes. In addition, an unusual segregation pattern for one locus followed by one probe may indicate homeologous chromosome pairing and segregation.     

Polyploids with different origins and ancestors form a single sexual polyploid species

Polyploidization is one of the few mechanisms that can produce instantaneous speciation. Multiple origins of tetraploid lineages from the same two diploid progenitors are common, but here we report the first known instance of a single tetraploid species that originated repeatedly from at least three diploid ancestors. Parallel evolution of advertisement calls in tetraploid lineages of gray tree frogs has allowed these lineages to interbreed, resulting in a single sexually interacting polyploid species despite the separate origins of polyploids from different diploids. Speciation by polyploidization in these frogs has been the source of considerable debate, but the various published hypotheses have assumed that polyploids arose through either autopolyploidy or allopolyploidy of extant diploid species. We utilized molecular markers and advertisement calls to infer the origins of tetraploid gray tree frogs. Previous hypotheses did not sufficiently account for the observed data. Instead, we found that tetraploids originated multiple times from extant diploid gray tree frogs and two other, apparently extinct, lineages of tree frogs. Tetraploid lineages then merged through interbreeding to result in a single species. Thus, polyploid species may have complex origins, especially in systems in which isolating mechanisms (such as advertisement calls) are affected directly through hybridization and polyploidy.     

Origins of polyploids: an example from peonies (Paeonia) and a model for angiosperms

The majority of tetraploid peonies are allopolyploids derived from crosses between phylogenetically distinct diploid lineages. Tetraploid Paeonia obovata was previously considered to be an autopolyploid because it is morphologically indistinguishable from the diploid of the same species. The presence of the Adh2 gene in tetraploid P. obovata but the inability to amplify the Adh2 gene from Chinese diploids of P. obovata, however, suggests that the tetraploid was not an autotetraploid derivative of the geographically adjacent diploid populations in China. The Adh gene phylogenies rather suggest that the tetraploid originated from crosses between two geographical races of diploid P. obovata distributed in China and Japan. The intermediate status of tetraploid P. obovata between auto‐ and allopolyploidy highlights the need for population genetic analyses of polyploid origins along the continuous range of genomic divergence. Here we present a model that describes the probabilities of polyploid formation and establishment as a function of genomic divergence between diploid progenitors. The probability of polyploid formation (P_f) is obtained from the multiplication of the probability of production of unreduced gametes (P_g) and the probability of ‘hybridization’ (P_h). P_f stays relatively stable when the genomic divergence is low, and then decreases progressively rapidly with the increase of genomic divergence between diploid progenitors. The probability of polyploid establishment (P_e), which depends on the rate of appearance of stable beneficial gene combinations and the rate of fertility restoration, is positively correlated with the genomic divergence of diploid parents. Multiplication of P_f and P_e gives an overall probability of polyploid origins (P_o) that varies continuously along the genomic divergence between diploid progenitors. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82 , 561–571.     

CHROMOSOMES AND CROSSING BEHAVIOR OF SOME SPECIES OF SANSEVIERIA

Menzel , Margaret Y. (Florida State U., Tallahassee), and James B. Pate . Chromosomes and crossing behavior of some species of Sansevieria. Amer. Jour. Bot. 47(3) : 230—238. Illus. 1960.–Approximately 20 species (28 clones) studied were diploids, tetraploids or hexaploids of the basic numbers x = 20; about 40% of the taxa were polyploid. All species had similar karyotypes, except for chromosome number. Five of 12 combinations of diploid species gave fertile F₁ hybrids; 4 studied cytologically showed 20 bivalents at metaphase I. Two triploid interspecific hybrids showed high trivalent frequencies. In contrast, multivalent formation in polyploid species was variable but rather low. Morphological relationships appeared reticulate among and between diploids and polyploids and did not coincide with barriers to crossing or to hybrid fertility. The following tentative hypothesis concerning relationships in the genus is proposed: Sansevieria is monophyletic and speciation has proceeded through genetic variation and hybridization at the diploid level and by allopolyploidy (of the segmental type) ; a low level of chromosome differentiation has accompanied speciation such that complete pairing occurs in diploid hybrids, but considerable preferential pairing occurs in allopolyploids. The occurrence of both polyploid and hybrid vigor, the fertility of hybrids between species differing greatly in morphology and physiology, and the high potential for vegetative propagation make the genus a favorable subject for breeding based on interspecific hybridization.     

EXTENSIVE GENE DUPLICATIONS IN DIPLOID EUPATORIUM (ASTERACEAE)

An electrophoretic study of isozyme number for seven soluble enzymes revealed extensive gene duplications in eight diploid species of American Eupatorium belonging to three morphological groups. The enzymes isocitrate dehydrogenase, phosphoglucomutase, phosphoglucose isomerase, 6-phosphogluconate dehydrogenase, and shikimate dehydrogenase occur as three to six isozymes in all species, whereas the minimal conserved number typical of diploid plants is two isozymes for each. Fructose 1, 6-biphosphate aldolase is expressed as multibanded pattern suggesting fixed heterozygosity in all examined species. It was not possible to document gene duplication for triosephosphate isomerase from the electrophoretic patterns. All species examined have a chromosome number of 2n = 20, which has been regarded as the basic diploid number for Eupatorium. However, the detection of extensive duplications suggests that 2n = 10 may be the original diploid chromosome number in Eupatorium and that plants with 2n = 20 are of polyploid origin. This hypothesis would mean that extensive duplications at isozyme gene loci have been maintained since the origin of the genus, despite chromosomal diploidization having occurred.     

Origins and introgression of polyploid species in Mentzelia section Trachyphytum (Loasaceae)

? Premise of the study: Polyploid speciation has been important in plant evolution. However, the conditions that favor the origination and persistence of polyploids are still not well understood. Here, we examine origins of 16 polyploid species in Mentzelia section Trachyphytum. ? Methods: We used phylogeny reconstructions based on DNA sequences from plastid regions and the nuclear gene isocitrate dehydrogenase (idh) to construct hypotheses of introgression and polyploidization. ? Key results: Molecular data suggest that homoploid hybridization has been surprisingly common in Trachyphytum. Diploid species had unequal involvement in polyploid origins, but most polyploid taxa had allopolyploid origins from extant progenitors. A few polyploids with extreme phenotypes did not appear to have extant progenitors. We infer that the progenitors of these species were derived from extinct diploid lineages or ancestral lineages of multiple extant diploids. In agreement with other recent studies, we recovered molecular evidence of multiple phylogenetically distinct origins for several polyploid taxa, including the widespread octoploid M. albicaulis. ? Conclusions: Evidence of high levels of introgression and allopolyploidy suggests that hybridization has played an important role in the evolution of Trachyphytum. Although idh sequences exhibited complicated evolution, including gene duplication, deletion, and recombination, they provided a higher percentage of informative characters for phylogeny reconstruction than the most variable plastid regions, allowing tests of hypotheses regarding polyploid origins. Given the necessity for rapidly evolving low-copy nuclear genes, researchers studying hybridization and polyploidy may increasingly turn to complex sequence data.     

CHROMOSOMES OF GRAPTOPETALUM AND THOMPSONELLA (CRASSULACEAE)

Chromosome numbers are reported for probably all 11 species of Graptopetalum (x = 30–35) and for both species of Thompsonella (x = 26). Plants of two species of Graptopetalum have gametic numbers from about 240–275, more than have been reported in any other seed plants. In hybrids the 30–35 chromosomes in the basic genome of Graptopetalum and likewise the 26 in Thompsonella apparently do not pair among themselves, and the genomes seem to be no more potent genetically than those of other species in their subfamily having as few as 12 chromosomes. Species with these gametic numbers are therefore considered to be diploid. On the other hand, in hybrids between a diploid and a plant with a very high chromosome number the phenotype of the latter predominates, and most of its chromosomes pair with each other. Many such hybrids are fertile. These facts suggest that the high polyploids arose by autoploidy rather than by alloploidy. Nevertheless, they may store heterozygosity at some gene loci and release it in various dosages and proportions each generation.     

Amphimixis and pseudogamy in fresh-water triclads: Experimental reconstitution of polyploid pseudogamic biotypes

In the fresh-water planarians Dugesia benazzii and D. lugubris, diploid amphimictic and polyploid pseudogamic biotypes occur, each of them with a peculiar chromosome cycle. In crosses between diploid biotype (acting as female) and polyploid ones, various degrees may be observed from amphimictic diploid to polyploid pseudogamic offspring. All cytological mechanisms that characterise the cycles of natural biotypes and of hybrids are inherited independently and only rarely can they be found together in a harmonic complex in such a way as to reconstruct the natural polyploid pseudogamic biotypes. The author analyses the genetical mechanisms which, in nature, may have led to the formation of these polyploid biotypes.Dedicated to Professor J. Seiler on the occasion of his 80th birthday.     

Reproductive strategies of diploid and polyploid populations of arcticDraba (Brassicaceae)

132 cultivated populations (2x–16x) of 15 arctic-alpine species ofDraba were investigated to clarify a possible relationship between reproductive strategies and polyploid evolution in the genus. The populations were exclusively sexual and produced viable seed after spontaneous self-pollination, but showed large variation both in traits promoting cross-pollination and in autogamous fruit and seed set. Traits promoting cross-pollination, e.g., floral display, protogyny, and delayed selfing, were positively correlated, and these traits were negatively correlated with autogamous fruit and seed set. All diploid and many polyploid populations had high autogamous seed set and small, unscented, non-protogynous, and rapidly selfing flowers. In contrast, all populations with low autogamous seed set and large, scented, and strongly protogynous flowers with distinctly delayed selfing were polyploid. These results are consistent with those previously obtained from enzyme electrophoresis, suggesting that the genetically depauperate diploids are extreme inbreeders and that the highly fixed-heterozygous polyploids vary from extreme inbreeders to mixed maters. The reproductive data lend additional support to the hypothesis that allopolyploidy in arcticDraba serves as an escape from genetic depauperation caused by uniparental inbreeding at the diploid level.     

Genome evolution in Oryza allopolyploids of various ages: Insights into the process of diploidization

The prevalence and recurrence of whole-genome duplication in plants and its major role in evolution have been well recognized. Despite great efforts, many aspects of genome evolution, particularly the temporal progression of genomic responses to allopolyploidy and the underlying mechanisms, remain poorly understood. The rice genus Oryza consists of both recently formed and older allopolyploid species, representing an attractive system for studying the genome evolution after allopolyploidy. In this study, through screening BAC libraries and sequencing and annotating the targeted BAC clones, we generated orthologous genomic sequences surrounding the DEP1 locus, a major grain yield QTL in cultivated rice, from four Oryza polyploids of various ages and their likely diploid genome donors or close relatives. Based on sequenced DEP1 region and published data from three other genomic regions, we investigated the temporal evolutionary dynamics of four polyploid genomes at both genetic and expression levels. In the recently formed BBCC polyploid, Oryza minuta, genome dominance was not observed and its short-term responses to allopolyploidy are mainly manifested as a high proportion of homoeologous gene pairs showing unequal expression. This could partly be explained by parental legacy, rewiring of divergent regulatory networks and epigenetic modulation. Moreover, we detected an ongoing diploidization process in this genus, and suggest that the expression divergence driven by changes of selective constraint probably plays a big role in the long-term diploidization. These findings add novel insights into our understanding of genome evolution after allopolyploidy, and could facilitate crop improvements through hybridization and polyploidization.     

THE CYTOLOGY AND DERIVATION OF A TEMPERATURE-SENSITIVE MEIOTIC MUTANT IN THE FERN CERATOPTERIS

Meiotic mutants were obtained from an inbreeding program following a hybridization between two diploid species of the homosporous fern Ceratopteris. The mutants are characterized by high levels of asynapsis at high temperatures (27 C, 33 C) and by aberrant spindle function at low temperatures (18 C). Spore viabilities vary with temperature and are highest at 23 C. Occasional restitution during the first division results in the production of dyads of diploid spores, some of which are viable. The inbreeding program was characterized by the expression of high levels of apparent heterozygosity in spite of intragametophytic selfing. It appears that segregation within the inbreeding program was responsible for the production of the mutants. The apparent heterozgosity and segregation can be explained by a previously documented genetic system within the polyploid homosporous ferns.     

NONHOMOLOGOUS CHROMOSOME PAIRING IN THE FERN CERATOPTERIS

The homosporous fern Ceratopteris thalictroides forms completely homozygous offspring sporophytes in one generation of selfing. Such homozygotes do not breed true and can exhibit structural heterozygosity for chromosomal mutations in some meiocytes. This is due to occasional nonhomologous (presumably homoeologous) chromosome pairing at meiosis in these polyploid ferns.     

Linked by Ancestral Bonds: Multiple Whole-Genome Duplications and Reticulate Evolution in a Brassicaceae Tribe

Pervasive hybridization and whole-genome duplications (WGDs) influenced genome evolution in several eukaryotic lineages. Although frequent and recurrent hybridizations may result in reticulate phylogenies, the evolutionary events underlying these reticulations, including detailed structure of the ancestral diploid and polyploid genomes, were only rarely reconstructed. Here, we elucidate the complex genomic history of a monophyletic clade from the mustard family (Brassicaceae), showing contentious relationships to the early-diverging clades of this model plant family. Genome evolution in the crucifer tribe Biscutelleae (∼60 species, 5 genera) was dominated by pervasive hybridizations and subsequent genome duplications. Diversification of an ancestral diploid genome into several divergent but crossable genomes was followed by hybridizations between these genomes. Whereas a single genus (Megadenia) remained diploid, the four remaining genera originated by allopolyploidy (Biscutella, Lunaria, Ricotia) or autopolyploidy (Heldreichia). The contentious relationships among the Biscutelleae genera, and between the tribe and other early diverged crucifer lineages, are best explained by close genomic relatedness among the recurrently hybridizing ancestral genomes. By using complementary cytogenomics and phylogenomics approaches, we demonstrate that the origin of a monophyletic plant clade can be more complex than a parsimonious assumption of a single WGD spurring postpolyploid cladogenesis. Instead, recurrent hybridization among the same and/or closely related parental genomes may phylogenetically interlink diploid and polyploid genomes despite the incidence of multiple independent WGDs. Our results provide new insights into evolution of early-diverging Brassicaceae lineages and elucidate challenges in resolving the contentious relationships within and between land plant lineages with pervasive hybridization and WGDs.     

Diploidization and genome size change in allopolyploids is associated with differential dynamics of low‐ and high‐copy sequences

Recent advances have highlighted the ubiquity of whole‐genome duplication (polyploidy) in angiosperms, although subsequent genome size change and diploidization (returning to a diploid‐like condition) are poorly understood. An excellent system to assess these processes is provided by Nicotiana section Repandae, which arose via allopolyploidy (approximately 5 million years ago) involving relatives of Nicotiana sylvestris and Nicotiana obtusifolia. Subsequent speciation in Repandae has resulted in allotetraploids with divergent genome sizes, including Nicotiana repanda and Nicotiana nudicaulis studied here, which have an estimated 23.6% genome expansion and 19.2% genome contraction from the early polyploid, respectively. Graph‐based clustering of next‐generation sequence data enabled assessment of the global genome composition of these allotetraploids and their diploid progenitors. Unexpectedly, in both allotetraploids, over 85% of sequence clusters (repetitive DNA families) had a lower abundance than predicted from their diploid relatives; a trend seen particularly in low‐copy repeats. The loss of high‐copy sequences predominantly accounts for the genome downsizing in N. nudicaulis. In contrast, N. repanda shows expansion of clusters already inherited in high copy number (mostly chromovirus‐like Ty3/Gypsy retroelements and some low‐complexity sequences), leading to much of the genome upsizing predicted. We suggest that the differential dynamics of low‐ and high‐copy sequences reveal two genomic processes that occur subsequent to allopolyploidy. The loss of low‐copy sequences, common to both allopolyploids, may reflect genome diploidization, a process that also involves loss of duplicate copies of genes and upstream regulators. In contrast, genome size divergence between allopolyploids is manifested through differential accumulation and/or deletion of high‐copy‐number sequences.     

EVOLUTION OF ASEXUALITY IN THE COSMOPOLITAN MARINE CLAM LASAEA

The marine clam genus Lasaea is unique among marine bivalves in that it contains both sexual and asexual lineages. We employed molecular tools to infer intrageneric relationships of geographically restricted sexual versus cosmopolitan asexual forms. Polymerase chain reaction primers were used to amplify and sequence homologous 624 nucleotide fragments of COIII from polyploid, asexual, direct-developing individuals representing northeastern Pacific, northeastern Atlantic, Mediterranean, southern Indian Ocean, and Australian populations. DNA sequences also were obtained from the two known diploid congeners, the Australian sexual, indirect developer, Lasaea australis, and an undescribed meiotic Australian direct developer. Estimated tree topologies did not support monophyly for polyploid asexual Lasaea lineages. A robust dichotomy was evident in all phylogenetic trees and each of the two main branches included one of the diploid meitoic Australian congeners. Lasaea australis clustered with two of the direct-developing, polyploid asexual haplotypes, one from Australia, the other from the northeastern Atlantic. Monophyly is supported for the diploid Australian direct-developing lineage together with the remaining polyploid asexual lineages from the northeastern Pacific, northeastern Atlantic, Mediterranean, and southern Indian Ocean. These results indicate that asexual Lasaea lineages are polyphyletic and may have resulted from multiple hybridization events. The high degree of genetic divergence of asexual lineages from co-clustering meiotic congeners (16%–22%) and among geographically restricted monophyletic clones (9%–11%) suggests that asexual Lasaea lineages may be exceptionally long lived.     

Salt‐tolerance diversity in diploid and polyploid cotton (Gossypium) species

The development of salt‐tolerant genotypes is pivotal for the effective utilization of salinized land and to increase global crop productivity. Several cotton species comprise the most important source of textile fibers globally, and these are increasingly grown on marginal or increasingly saline agroecosystems. The allopolyploid cotton species also provide a model system for polyploid research, of relevance here because polyploidy was suggested to be associated with increased adaptation to stress. To evaluate the genetic variation of salt tolerance among cotton species, 17 diverse accessions of allopolyploid (AD‐genome) and diploid (A‐ and D‐genome) Gossypium were evaluated for a total of 29 morphological and physiological traits associated with salt tolerance. For most morphological and physiological traits, cotton accessions showed highly variable responses to 2 weeks of exposure to moderate (50 mm NaCl) and high (100 mm NaCl) hydroponic salinity treatments. Our results showed that the most salt‐tolerant species were the allopolyploid Gossypium mustelinum from north‐east Brazil, the D‐genome diploid Gossypium klotzschianum from the Galapagos Islands, followed by the A‐genome diploids of Africa and Asia. Generally, A‐genome accessions outperformed D‐genome cottons under salinity conditions. Allopolyploid accessions from either diploid genomic group did not show significant differences in salt tolerance, but they were more similar to one of the two progenitor lineages. Our findings demonstrate that allopolyploidy in itself need not be associated with increased salinity stress tolerance and provide information for using the secondary Gossypium gene pool to breed for improved salt tolerance.     

Genome‐wide sequence information reveals recurrent hybridization among diploid wheat wild relatives

Many conflicting hypotheses regarding the relationships among crops and wild species closely related to wheat (the genera Aegilops, Amblyopyrum, and Triticum) have been postulated. The contribution of hybridization to the evolution of these taxa is intensely discussed. To determine possible causes for this, and provide a phylogeny of the diploid taxa based on genome‐wide sequence information, independent data were obtained from genotyping‐by‐sequencing and a target‐enrichment experiment that returned 244 low‐copy nuclear loci. The data were analyzed using Bayesian, likelihood and coalescent‐based methods. D statistics were used to test if incomplete lineage sorting alone or together with hybridization is the source for incongruent gene trees. Here we present the phylogeny of all diploid species of the wheat wild relatives. We hypothesize that most of the wheat‐group species were shaped by a primordial homoploid hybrid speciation event involving the ancestral Triticum and Am. muticum lineages to form all other species except Ae. speltoides. This hybridization event was followed by multiple introgressions affecting all taxa except Triticum. Mostly progenitors of the extant species were involved in these processes, while recent interspecific gene flow seems insignificant. The composite nature of many genomes of wheat‐group taxa results in complicated patterns of diploid contributions when these lineages are involved in polyploid formation, which is, for example, the case for tetraploid and hexaploid wheats. Our analysis provides phylogenetic relationships and a testable hypothesis for the genome compositions in the basic evolutionary units within the wheat group of Triticeae.     

Is genome downsizing associated with diversification in polyploid lineages of Veronica?

The study of genome size evolution in a phylogenetic context in related polyploid and diploid lineages can help us to understand the advantages and disadvantages of genome size changes and their effect on diversification. Here, we contribute 199 new DNA sequences and a nearly threefold increase in genome size estimates in polyploid and diploid Veronica (Plantaginaceae) (to 128 species, c. 30% of the genus) to provide a comprehensive baseline to explore the effect of genome size changes. We reconstructed internal transcribed spacer (ITS) and trnL‐trnL‐trnF phylogenetic trees and performed phylogenetic generalized least squares (PGLS), ancestral character state reconstruction, molecular dating and diversification analyses. Veronica 1C‐values range from 0.26 to 3.19 pg. Life history is significantly correlated with 1C‐value, whereas ploidy and chromosome number are strongly correlated with both 1C‐ and 1Cx‐values. The estimated ancestral Veronica 1Cx‐value is 0.65 pg, with significant genome downsizing in the polyploid Southern Hemisphere subgenus Pseudoveronica and two Northern Hemisphere subgenera, and significant genome upsizing in two diploid subgenera. These genomic downsizing events are accompanied by increased diversification rates, but a ‘core shift’ was only detected in the rate of subgenus Pseudoveronica. Polyploidy is important in the evolution of the genus, and a link between genome downsizing and polyploid diversification and species radiations is hypothesized. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 178 , 243–266.