Nicotiana Section Research Articles

Nine new species of Australian Nicotiana (Solanaceae)

Nine new species in Australian Nicotiana section Suaveolentes are described, including N. clarksonii M.W.Chase & Christenh., N. erytheia M.W.Chase & Christenh., N. latifolia M.W.Chase & Christenh., N. latzii M.W.Chase, R.W.Jobson & Christenh., N. gibbosa M.W.Chase, D.D.Andrew & J.J.Bruhl, N. olens M.W.Chase & Christenh., N. praecipitis M.W.Chase & K.Durham, N. karara M.W.Chase & Christenh. and N. bungonia M.W.Chase & Taseski. Some have been known from herbarium specimens for a long time, but their distinction from other species concepts was unsuspected until DNA studies showed their divergent nature. Others are known from one or only a few new localities. All are morphologically and genetically distinct from their close relatives. Increased sampling of populations in our molecular studies has led us to the conclusion that the widespread, recently described species N. insecticida is a species complex, comprising as many as six genetically distinct taxa, one of which includes material from the type locality of N. hesperis. Here, we describe a new name, N. erytheia, for all material we previously assigned to N. hesperis that is phylogenetically distinct from that of the type locality. To make the relationships of the new species clearer, we provide a tree produced by analysis of nuclear (RADseq) DNA data.

Polyploid Nicotiana section Suaveolentes originated by hybridization of two ancestral Nicotiana clades.

Nicotiana section Suaveolentes is an almost all-Australian clade of allopolyploid tobacco species that emerged through hybridization between diploid relatives of the genus. In this study, we aimed to assess the phylogenetic relationship of the Suaveolentes section with several Nicotiana diploid species based on both plastidial and nuclear genes. The Nicotiana plastome-based phylogenetic analysis representing 47 newly re-built plastid genomes suggested that an ancestor of N. section Noctiflorae is the most likely maternal donor of the Suaveolentes clade. Nevertheless, we found clear evidence of plastid recombination with an ancestor from the Sylvestres clade. We analyzed 411 maximum likelihood-based phylogenetic trees from a set of conserved nuclear diploid single copy gene families following an approach that assessed the genomic origin of each homeolog. We found that Nicotiana section Suaveolentes is monophyletic with contributions from the sections Alatae, Sylvestres, Petunioides and Noctiflorae. The dating of the divergence between these sections indicates that the Suaveolentes hybridization predates the split between Alatae/Sylvestres, and Noctiflorae/Petunioides. We propose that Nicotiana section Suaveolentes arose from the hybridization of two ancestral species from which the Noctiflorae/Petunioides and Alatae/Sylvestres sections are derived, with Noctiflorae the maternal parent. This study is a good example in which the use of genome wide data provided additional evidence about the origin of a complex polyploid clade.

Down, then up: non-parallel genome size changes and a descending chromosome series in a recent radiation of the Australian allotetraploid plant species, Nicotiana section Suaveolentes (Solanaceae).

The extent to which genome size and chromosome numbers evolve in concert is little understood, particularly after polyploidy (whole-genome duplication), when a genome returns to a diploid-like condition (diploidization). We study this phenomenon in 46 species of allotetraploid Nicotiana section Suaveolentes (Solanaceae), which formed <6 million years ago and radiated in the arid centre of Australia. We analysed newly assessed genome sizes and chromosome numbers within the context of a restriction site-associated nuclear DNA (RADseq) phylogenetic framework. RADseq generated a well-supported phylogenetic tree, in which multiple accessions from each species formed unique genetic clusters. Chromosome numbers and genome sizes vary from n = 2x = 15 to 24 and 2.7 to 5.8 pg/1C nucleus, respectively. Decreases in both genome size and chromosome number occur, although neither consistently nor in parallel. Species with the lowest chromosome numbers (n = 15-18) do not possess the smallest genome sizes and, although N. heterantha has retained the ancestral chromosome complement, n = 2x = 24, it nonetheless has the smallest genome size, even smaller than that of the modern representatives of ancestral diploids. The results indicate that decreases in genome size and chromosome number occur in parallel down to a chromosome number threshold, n = 20, below which genome size increases, a phenomenon potentially explained by decreasing rates of recombination over fewer chromosomes. We hypothesize that, more generally in plants, major decreases in genome size post-polyploidization take place while chromosome numbers are still high because in these stages elimination of retrotransposons and other repetitive elements is more efficient. Once such major genome size change has been accomplished, then dysploid chromosome reductions take place to reorganize these smaller genomes, producing species with small genomes and low chromosome numbers such as those observed in many annual angiosperms, including Arabidopsis.

Evaluation of Crossability between Nicotiana benthamiana and Nicotiana excelsior

Wild tobacco species in the Nicotiana section Suaveolentes are promising genetic resources to introduce their disease resistance to cultivated tobacco, Nicotiana tabacum. However, hybrid lethality is observed in hybrid seedlings from crosses between most Suaveolentes species and N. tabacum. In particular, N. benthamiana belonging to the section Suaveolentes produces only viable hybrids after crossing with N. tabacum. In the present study, crossability between N. benthamiana and N. excelsior (section Suaveolentes) was investigated to test the possible usefulness of N. benthamiana as the bridge parent to transfer desirable genes of N. excelsior to N. tabacum via bridge crossing. After reciprocal crosses using three accessions of N. benthamiana and N. excelsior each, several crossing barriers such as cross-incompatibility, seed abortion, and male and female hybrid sterility were observed. Although reciprocal hybrids between N. benthamiana and N. excelsior showed a high degree of chromosome pairing in meiosis, univalents and multivalents, as well as chromosome bridges and lagging chromosomes, were observed. These meiotic abnormalities were thought to cause hybrid sterility. The possible usefulness of reciprocal hybrids between N. benthamiana and N. excelsior is discussed.

Extensive plastid-nuclear discordance in a recent radiation ofNicotianasectionSuaveolentes(Solanaceae)

AbstractNicotiana section Suaveolentes is the largest section of Nicotiana and is a monophyletic group of allotetraploid species. Most of the species are endemic to Australia, but three species occur on islands in the South Pacific as far east as French Polynesia and one species is native to Namibia. Here, we present phylogenetic results based on genome skimming, with near-complete taxon sampling and multiple accessions sampled for several species. These represent the first phylogenetic results for the section that include most recognized taxa, using wild-sourced material wherever possible. Despite known chromosome number and genome size changes in the section, there is little divergence in the ribosomal DNA operon (26S, 18.S and 5.8S plus associated spacers) and plastid genomes, with little to no taxonomic signal in plastome phylogenetic results and clear plastid-nuclear discordance. These results contrast with strong morphological differentiation (both floral and vegetative) between most of the core Australian taxa and obvious differences in ecological preferences. Together, these initial results portray Nicotiana section Suaveolentes as experiencing recent and ongoing radiation in the arid zone of Australia.

Parental origin of the allotetraploid tobacco Nicotiana benthamiana.

Summary Nicotiana section Suaveolentes is an almost all‐Australian clade of allopolyploid tobacco species including the important plant model Nicotiana benthamiana. The homology relationships of this clade and its formation are not completely understood. To address this gap, we assessed phylogenies of all individual genes of N. benthamiana and the well studied N. tabacum (section Nicotiana) and their homologues in six diploid Nicotiana species. We generated sets of 44 424 and 65 457 phylogenetic trees of N. benthamiana and N. tabacum genes, respectively, each collectively called a phylome. Members of Nicotiana sections Noctiflorae and Sylvestres were represented as the species closest to N. benthamiana in most of the gene trees. Analyzing the gene trees of the phylome we: (i) dated the hybridization event giving rise to N. benthamiana to 4–5 MyA, and (ii) separated the subgenomes. We assigned 1.42 Gbp of the genome sequence to section Noctiflorae and 0.97 Gbp to section Sylvestres based on phylome analysis. In contrast, read mapping of the donor species did not succeed in separating the subgenomes of N. benthamiana. We show that the maternal progenitor of N. benthamiana was a member of section Noctiflorae, and confirm a member of section Sylvestres as paternal subgenome donor. We also demonstrate that the advanced stage of long‐term genome diploidization in N. benthamiana is reflected in its subgenome organization. Taken together, our results underscore the usefulness of phylome analysis for subgenome characterization in hybrid species.

Nicotiana suaveolens accessions with different ploidy levels exhibit different reproductive isolation mechanisms in interspecific crosses with Nicotiana tabacum.

Reproductive isolation, including prezygotic and postzygotic barriers, is a mechanism that separates species. Many species in the Nicotiana section Suaveolentes exhibit reproductive isolation in crosses with Nicotiana tabacum. In this study, we investigated whether the chromosome numbers and ploidy levels of eight Nicotiana suaveolens accessions are related to the reproductive isolation after crosses with N. tabacum by flow cytometry and chromosome analyses. Additionally, the internal transcribed spacer (ITS) regions of the eight N. suaveolens accessions were sequenced and compared with the previously reported sequences of 22 Suaveolentes species to elucidate the phylogenetic relationships in the section Suaveolentes. We revealed that four N. suaveolens accessions comprised 64 chromosomes, while the other four accessions carried 32 chromosomes. Depending on the ploidy levels of N. suaveolens, several types of reproductive isolation were observed after crosses with N. tabacum, including decreases in the number of capsules and the germination rates of hybrid seeds, as well as hybrid lethality and abscission of enlarged ovaries at 12-17days after pollination. A phylogenetic analysis involving ITS sequences divided the eight N. suaveolens accessions into three distinct clades. Based on the results, we confirmed that N. suaveolens accessions vary regarding ploidy levels and reproductive isolation mechanisms in crosses with N. tabacum. These accessions will be very useful for revealing and characterizing the reproductive isolation mechanisms in interspecific crosses and their relationships with ploidy levels.

Time-calibrated phylogenetic trees establish a lag between polyploidisation and diversification in Nicotiana (Solanaceae)

We investigate the timing of diversification in allopolyploids of Nicotiana (Solanaceae) utilising sequence data of maternal and paternal origin to look for evidence of a lag phase during which diploidisation took place. Bayesian relaxed clock phylogenetic methods show recent allopolyploids are a result of several unique polyploidisation events, and older allopolyploid sections have undergone subsequent speciation at the polyploid level (i.e. a number of these polyploid species share a singular origin). The independently formed recent polyploid species in the genus all have mean age estimates below 1 million years ago (Ma). Nicotiana section Polydicliae (two species) evolved 1.5 Ma, N. section Repandae (four species) formed 4 Ma, and N. section Suaveolentes (~35 species) is about 6 million years old. A general trend of higher speciation rates in older polyploids is evident, but diversification dramatically increases at approximately 6 Ma (in section Suaveolentes). Nicotiana sect. Suaveolentes has spectacularly radiated to form 35 species in Australia and some Pacific islands following a lag phase of almost 6 million years. Species have filled new ecological niches and undergone extensive diploidisation (e.g. chromosome fusions bringing the ancestral allotetraploid number, n = 24, down to n = 15 and ribosomal loci numbers back to diploid condition). Considering the progenitors of Suaveolentes inhabit South America, this represents the colonisation of Australia by polyploids that have subsequently undergone a recent radiation into new environments. To our knowledge, this study is the first report of a substantial lag phase being investigated below the family level.

Related allopolyploids display distinct floral pigment profiles and transgressive pigments.

Both polyploidy and shifts in floral color have marked angiosperm evolution. Here, we investigate the biochemical basis of the novel and diverse floral phenotypes seen in allopolyploids in Nicotiana (Solanaceae) and examine the extent to which the merging of distinct genomes alters flavonoid pigment production. We analyzed flavonol and anthocyanin pigments from Nicotiana allopolyploids of different ages (N. tabacum, 0.2 million years old; several species from Nicotiana section Repandae, 4.5 million years old; and five lines of first-generation synthetic N. tabacum) as well as their diploid progenitors. Allopolyploid floral pigment profiles tend not to overlap with their progenitors or related allopolyploids, and allopolyploids produce transgressive pigments that are not present in either progenitor. Differences in floral color among N. tabacum accessions seems mainly to be due to variation in cyanidin concentration, but changes in flavonol concentrations among accessions are also present. Competition for substrates within the flavonoid biosynthetic pathway to make either flavonols or anthocyanins may drive the differences seen among related allopolyploids. Some of the pigment differences observed in allopolyploids may be associated with making flowers more visible to nocturnal pollinators.

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.

Differential Dynamics of Transposable Elements during Long-Term Diploidization of Nicotiana Section Repandae (Solanaceae) Allopolyploid Genomes

Evidence accumulated over the last decade has shown that allopolyploid genomes may undergo drastic reorganization. However, timing and mechanisms of structural diploidization over evolutionary timescales are still poorly known. As transposable elements (TEs) represent major and labile components of plant genomes, they likely play a pivotal role in fuelling genome changes leading to long-term diploidization. Here, we exploit the 4.5 MY old allopolyploid Nicotiana section Repandae to investigate the impact of TEs on the evolutionary dynamics of genomes. Sequence-specific amplified polymorphisms (SSAP) on seven TEs with expected contrasted dynamics were used to survey genome-wide TE insertion polymorphisms. Comparisons of TE insertions in the four allopolyploid species and descendents of the diploid species most closely related to their actual progenitors revealed that the polyploids showed considerable departure from predicted additivity of the diploids. Large numbers of new SSAP bands were observed in polyploids for two TEs, but restructuring for most TE families involved substantial loss of fragments relative to the genome of the diploid representing the paternal progenitor, which could be due to changes in allopolyploids, diploid progenitor lineages or both. The majority of non-additive bands were shared by all polyploid species, suggesting that significant restructuring occurred early after the allopolyploid event that gave rise to their common ancestor. Furthermore, several gains and losses of SSAP fragments were restricted to N. repanda, suggesting a unique evolutionary trajectory. This pattern of diploidization in TE genome fractions supports the hypothesis that TEs are central to long-term genome turnover and depends on both TE and the polyploid lineage considered.

RECONSTRUCTING THE COMPLEX EVOLUTIONARY ORIGIN OF WILD ALLOPOLYPLOID TOBACCOS (NICOTIANASECTIONSUAVEOLENTES)

Nicotiana (Solanaceae) provides an ideal system for understanding polyploidization, a pervasive and powerful evolutionary force in plants, as this genus contains several groups of allotetraploids that formed at different times from different diploid progenitors. However, the parental lineages of the largest group of allotetraploids, Nicotiana section Suaveolentes, have been problematic to identify. Using data from four regions of three low-copy nuclear genes, nuclear ribosomal DNA, and regions of the plastid genome, we have reconstructed the evolutionary origin of sect. Suaveolentes and identified the most likely diploid progenitors by using a combination of gene trees and network approaches to uncover the most strongly supported evidence of species relationships. Our analyses best support a scenario where a member of the sect. Sylvestres lineage acted as the paternal progenitor and a member of either sect. Petunioides or sect. Noctiflorae that also contained introgressed DNA from the other, or a hypothetical hybrid species between these two sections, was the maternal progenitor. Nicotiana exemplifies many of the factors that can complicate the reconstruction of polyploid evolutionary history and highlights how reticulate evolution at the diploid level can add even greater complexity to allopolyploid genomes.

Root-to-shoot Translocation of Alkaloids is Dominantly Suppressed in Nicotiana alata

In tobacco (Nicotiana tabacum), nicotine and related pyridine alkaloids are produced in the root, and then transported to the aerial parts where these toxic chemicals function as part of chemical defense against insect herbivory. Although a few tobacco transporters have been recently reported to take up nicotine into the vacuole from the cytoplasm or into the cytoplasm from the apoplast, it is not known how the long-range translocation of tobacco alkaloids between organs is controlled. Nicotiana langsdorffii and N. alata are closely related species of diploid Nicotiana section Alatae, but the latter does not accumulate tobacco alkaloids in the leaf. We show here that N. alata does synthesize alkaloids in the root, but lacks the capacity to mobilize the root-borne alkaloids to the aerial parts. Interspecific grafting experiments between N. alata and N. langsdorffii indicate that roots of N. alata are unable to translocate alkaloids to their shoot system. Interestingly, genetic studies involving interspecific hybrids between N. alata and N. langsdorffii and their self-crossed or back-crossed progeny showed that the non-translocation phenotype is dominant over the translocation phenotype. These results indicate that a mechanism to retain tobacco alkaloids within the root organ has evolved in N. alata, which may represent an interesting strategy to control the distribution of secondary products within a whole plant.

Genes in S and T Subgenomes Are Responsible for Hybrid Lethality in Interspecific Hybrids between Nicotiana tabacum and Nicotiana occidentalis

BackgroundMany species of Nicotiana section Suaveolentes produce inviable F1 hybrids after crossing with Nicotiana tabacum (genome constitution SSTT), a phenomenon that is often called hybrid lethality. Through crosses with monosomic lines of N. tabacum lacking a Q chromosome, we previously determined that hybrid lethality is caused by interaction between gene(s) on the Q chromosome belonging to the S subgenome of N. tabacum and gene(s) in Suaveolentes species. Here, we examined if hybrid seedlings from the cross N. occidentalis (section Suaveolentes)×N. tabacum are inviable despite a lack of the Q chromosome.Methodology/Principal FindingsHybrid lethality in the cross of N. occidentalis×N. tabacum was characterized by shoots with fading color. This symptom differed from what has been previously observed in lethal crosses between many species in section Suaveolentes and N. tabacum. In crosses of monosomic N. tabacum plants lacking the Q chromosome with N. occidentalis, hybrid lethality was observed in hybrid seedlings either lacking or possessing the Q chromosome. N. occidentalis was then crossed with two progenitors of N. tabacum, N. sylvestris (SS) and N. tomentosiformis (TT), to reveal which subgenome of N. tabacum contains gene(s) responsible for hybrid lethality. Hybrid seedlings from the crosses N. occidentalis×N. tomentosiformis and N. occidentalis×N. sylvestris were inviable.Conclusions/SignificanceAlthough the specific symptoms of hybrid lethality in the cross N. occidentalis×N. tabacum were similar to those appearing in hybrids from the cross N. occidentalis×N. tomentosiformis, genes in both the S and T subgenomes of N. tabacum appear responsible for hybrid lethality in crosses with N. occidentalis.

Corolla shape vs. size in flower choice by a nocturnal hawkmoth pollinator

1. Corolla shape is one of many floral traits used by pollinators when making foraging decisions. Corolla shape has been implicated as a factor in floral isolation and evolutionary shifts between pollinator affinities, yet few studies have investigated how quantifiable differences in corolla shape affect pollinator behaviour. 2. This study measured variation in corolla shape (primarily corolla limb dissectedness) across eight species of Nicotiana (section Alatae) and its influence on flower choice by nocturnal hawkmoths (Manduca sexta). Behavioural assays performed under simulated moonlight tested for hawkmoth preference and performance differences between the average floral shape of sphingophilous species (CHawkmoth), the average of all Nicotiana species in this study (CAll), and/or the average of hummingbird-pollinated species (CHummingbird). 3. In mixed arrays, hawkmoths preferred CHawkmoth over CAll flowers, but only when shapes were not standardized for surface area. In homogeneous arrays, hawkmoths emptied more flowers with CHawkmoth shapes, but performance did not differ when shapes were standardized by surface area. However, hawkmoths visited fewer CHummingbird flowers than other shapes when surface area was standardized. 4. Our study shows that hawkmoth behaviour can be affected by differences in corolla shape, but that floral size may be more important than true shape differences in floral discrimination by hawkmoths.

Molecular Linkage Mapping and Marker-Trait Associations with NlRPT, a Downy Mildew Resistance Gene in Nicotiana langsdorffii.

Nicotiana langsdorffii is one of two species of Nicotiana known to express an incompatible interaction with the oomycete Peronospora tabacina, the causal agent of tobacco blue mold disease. We previously showed that incompatibility is due to the hypersensitive response (HR), and plants expressing the HR are resistant to P. tabacina at all stages of growth. Resistance is due to a single dominant gene in N. langsdorffii accession S-4-4 that we have named NlRPT. In further characterizing this unique host-pathogen interaction, NlRPT has been placed on a preliminary genetic map of the N. langsdorffii genome. Allelic scores for five classes of DNA markers were determined for 90 progeny of a “modified backcross” involving two N. langsdorffii inbred lines and the related species N. forgetiana. All markers had an expected segregation ratio of 1:1, and were scored in a common format. The map was constructed with JoinMap 3.0, and loci showing excessive transmission distortion were removed. The linkage map consists of 266 molecular marker loci defined by 217 amplified fragment length polymorphisms (AFLPs), 26 simple-sequence repeats (SSRs), 10 conserved orthologous sequence markers, nine inter-simple sequence repeat markers, and four target region amplification polymorphism markers arranged in 12 linkage groups with a combined length of 1062 cM. NlRPT is located on linkage group three, flanked by four AFLP markers and one SSR. Regions of skewed segregation were detected on LGs 1, 5, and 9. Markers developed for N. langsdorffii are potentially useful genetic tools for other species in Nicotiana section Alatae, as well as in N. benthamiana. We also investigated whether AFLPs could be used to infer genetic relationships within N. langsdorffii and related species from section Alatae. A phenetic analysis of the AFLP data showed that there are two main lineages within N. langsdorffii, and that both contain populations expressing dominant resistance to P. tabacina.

Karyotypic variation in Nicotiana section Suaveolentes

Nicotiana section Suaveolentes (Solanaceae) currently includes 28 species and subspecies that are endemic to Australasia and the South Pacific and one African species, N. africana. The section is monophyletic and of allotetraploid origin, but relationships among the species in it and its diploid progenitors are poorly understood. Here we report chromosome numbers for 20 of the 29 taxa from the Suaveolentes, including a count for one recently proposed species for which no number has previously been available. Many of the published chromosome numbers for the Suaveolentes are confirmed in this study. However, six counts were different from the published numbers including n = 15 for N. maritima and N. suaveolens, which is a new chromosome number for the genus. Nicotiana goodspeedii and N. rotundifolia were n = 16, and the same number was found in the suggested species N. sp. ‘Corunna’. Nicotiana suaveolens contains polyploid races of n = 32 and here we report the probable existence of an n = 31 race as well. Karyotypic variation within species and within the section is apparently much greater than previously thought and further investigation is warranted.

Biogeography of Nicotiana section Suaveolentes (Solanaceae) reveals geographical tracks in arid Australia

Aim Nicotiana section Suaveolentes is largely endemic to Australia but includes one species endemic to Africa, one to New Caledonia and Tongatupa, and one to the Marquesas Islands in the Pacific. Other sections of Nicotiana are found in the New World. In Australia, Suaveolentes is widespread across the continent, with many taxa adapted to the Eremean zone. We aim to analyse the biogeography of the Australian clade, both to shed light on the evolution of the group and to determine general area relationships that provide insight into the history of the arid-zone biota. Location Mesic and arid regions of continental Australia, the Central–South Pacific and Namibia, Africa. Methods A phylogeny of Suaveolentes, based on morphology and molecular data, was used to analyse the relationships of areas in which the taxa occur. The section is monophyletic, and all but three taxa were included (25). The method of paralogy-free subtree analysis was employed, with the basal taxon Nicotiana africana used as the outgroup. Results Paralogy-free subtree analysis found five area subtrees that, when combined, resulted in a minimal area cladogram with six resolved nodes. Pacific and mesic eastern Australia (including Lord Howe Island) are at the base of the area cladogram, followed by the differentiation of North West Australia and later South East Australia. Arid regions of Australia are related, revealing three biogeographical tracks: a northern track including the Great Sandy Desert and Tanami, which are related to the Pilbara; a central track relating the Western Desert, Central Ranges, Eastern Desert and North East Interzone; and a southern track relating the South West Interzone, Nullarbor, Adelaide/Eyre and the South East Interzone. Plesiomorphic taxa with chromosome number n = 24–23 occur on the periphery of the continent, and derived taxa with n = 21, 20, 18, 16–15 identify the tracks across arid Australia. Main conclusions The patterns of distribution and differentiation of Suaveolentes in Australia show that the age of the clade is at least Early Miocene, dating to before the onset of aridification in Australia about 15 Ma. The patterns are also interpreted as evidence that it was vicariance that largely shaped speciation in the Eremean zone, with range expansion of some widespread taxa probably occurring in the most recent cycles of severe drying and mobilization of desert dune sands.

Seven of eight species in Nicotiana section Suaveolentes have common factors leading to hybrid lethality in crosses with Nicotiana tabacum

Reproductive isolation is a mechanism that separates species, and is classified into two types: prezygotic and postzygotic. Inviability of hybrids, or hybrid lethality, is a type of postzygotic isolation and is observed in some plant species, including Nicotiana species. Previous work has shown that the Q chromosome, which belongs to the S subgenome of N. tabacum, encodes one or more genes leading to hybrid lethality in some crosses. Interspecific crosses of eight wild species were conducted in section Suaveolentes (which consists of species restricted to Australasia and Africa) with the cultivated species Nicotiana tabacum. Hybrid seedlings were cultivated at 28, 34 or 36 degrees C, and PCR and chromosome analysis were performed. Seven of eight wild species produced inviable hybrids after crossing. Hybrid lethality, which was observed in all crosses at 28 degrees C, was Type II lethality, with the characteristic symptoms of browning of hypocotyl and roots; lethality was suppressed at elevated temperatures (34 or 36 degrees C). Furthermore, one or more genes on the Q chromosome of N. tabacum were absolutely responsible for hybrid lethality, suggesting that many species of section Suaveolentes share the same factor that triggers hybrid lethality by interaction with the genes on the Q chromosome. Exceptionally, only one wild species, N. fragrans, produced 100 % viable hybrids after crossing with N. tabacum, suggesting that N. fragrans has no factor triggering hybrid lethality.

Fall and rise of satellite repeats in allopolyploids of Nicotiana over c. 5 million years

Allopolyploids represent natural experiments in which DNA sequences from different species are combined into a single nucleus and then coevolve, enabling us to follow the parental genomes, their interactions and evolution over time. Here, we examine the fate of satellite DNA over 5 million yr of divergence in plant genus Nicotiana (family Solanaceae). We isolated subtelomeric, tandemly repeated satellite DNA from Nicotiana diploid and allopolyploid species and analysed patterns of inheritance and divergence by sequence analysis, Southern blot hybridization and fluorescent in situ hybridization (FISH). We observed that parental satellite sequences redistribute around the genome in allopolyploids of Nicotiana section Polydicliae, formed c. 1 million yr ago (Mya), and that new satellite repeats evolved and amplified in section Repandae, which was formed c. 5 Mya. In some cases that process involved the complete replacement of parental satellite sequences. The rate of satellite repeat replacement is faster than theoretical predictions assuming the mechanism involved is unequal recombination and crossing-over. Instead we propose that this mechanism occurs with the deletion of large chromatin blocks and reamplification, perhaps via rolling circle replication.