Pachycladon Species Research Articles

Linkage between trichome morphology and leaf optical properties in New Zealand alpine Pachycladon (Brassicaceae)

Trichomes are important regulators of leaf radiation absorptance and thus modulate the carbon, water and energy balance of pubescent plants. This study assessed leaf hair characteristics and leaf optical properties of four species of alpine Pachycladon (Brassicaceae). Two of the pubescent Pachycladon species exhibited different trichome morphological characteristics but only small variations in leaf hair density. These structural differences explained much of the variation in optical properties between species. Pachycladon stellatum exhibited the highest proportion of dendritic trichomes (87%), with forked and simple trichomes more common in the other species. This high proportion of dendritic trichomes contributed to the c. 8.3% reflectance in physiologically important regions of the electromagnetic spectrum in P. stellatum leaves compared with c. 6.5% reflectance in P. enysii leaves, which showed similar leaf hair density but fewer dendritic trichomes (7%). Together, these data indicate an under-appreciated role for trichome morphology in regulating leaf absorption of radiation in Pachycladon.

Chips and tags suggest plant-environment interactions differ for two alpine Pachycladon species

BackgroundExpression profiling has been proposed as a means for screening non-model organisms in their natural environments to identify genes potentially important in adaptive diversification. Tag profiling using high throughput sequencing is a relatively low cost means of expression profiling with deep coverage. However the extent to which very short cDNA sequences can be effectively used in screening for candidate genes is unclear. Here we investigate this question using an evolutionarily distant as well as a closely related transcriptome for referencing tags. We do this by comparing differentially expressed genes and processes between two closely related allopolyploid species of Pachycladon which have distinct altitudinal preferences in the New Zealand Southern Alps. We validate biological inferences against earlier microarray analyses.ResultsStatistical and gene annotation enrichment analyses of tag profiles identified more differentially expressed genes of potential adaptive significance than previous analyses of array-based expression profiles. These include genes involved in glucosinolate metabolism, flowering time, and response to cold, desiccation, fungi and oxidation. In addition, despite the short length of 20mer tags, we were able to infer patterns of homeologous gene expression for 700 genes in our reference library of 7,128 full-length Pachycladon ESTs. We also demonstrate that there is significant information loss when mapping tags to the non-conspecific reference transcriptome of A. thaliana as opposed to P. fastigiatum ESTs but also describe mapping strategies by which the larger collection of A. thaliana ESTs can be used as a reference.ConclusionWhen coupled with a reference transcriptome generated using RNA-seq, tag sequencing offers a promising approach for screening natural populations and identifying candidate genes of potential adaptive significance. We identify computational issues important for the successful application of tag profiling in a non-model allopolyploid plant species.

Shotgun proteomic profiling of five species of New Zealand Pachycladon

AbstractThe genus Pachycladon consists of ten species of alpine plants, nine of which are endemic to New Zealand. The species are closely related to the model plant Arabidopsis thaliana with respect to their sequence divergence and chromosome synteny, occupy distinct geographical habitats in terms of both latitude and altitude, and display a range of morphologies. We have performed label‐free quantitative shotgun proteomic analysis of five different species of Pachycladon, namely P. cheesemanii (CH), P. exile (EX), P. fastigiatum (FA), P. enysii (EN) and P. novae‐zelandiae (NZ). The total non‐redundant data set for all five species contained 1489 proteins. The numbers of proteins identified reproducibly in each species ranged from 629 for CH to 987 for NZ, with 681 for EN, 741 for EX and 934 for FA. Previous metabolite‐based studies have shown that FA hydrolyzes glucosinolates completely to isothiocyanates while EN converts glucosinolates to nitriles. In this study, we observed high expression of ESP (At1g54040, epithiospecifying senescence regulator protein) and myrosinase 2 (At5g25980, glycosyl hydrolase family protein), which result in production of nitriles and epithionitriles, in EN and NZ, and we also observed higher expression of ESM1 (At3g14210, GDSL esterase/lipase), which mediates the formation of isothiocyanate, in FA.

Island species radiation and karyotypic stasis in Pachycladonallopolyploids

BackgroundPachycladon (Brassicaceae, tribe Camelineae) is a monophyletic genus of ten morphologically and ecogeographically differentiated, and presumably allopolyploid species occurring in the South Island of New Zealand and in Tasmania. All Pachycladon species possess ten chromosome pairs (2n = 20). The feasibility of comparative chromosome painting (CCP) in crucifer species allows the origin and genome evolution in this genus to be elucidated. We focus on the origin and genome evolution of Pachycladon as well as on its genomic relationship to other crucifer species, particularly to the allopolyploid Australian Camelineae taxa. As species radiation on islands is usually characterized by chromosomal stasis, i.e. uniformity of chromosome numbers/ploidy levels, the role of major karyotypic reshuffling during the island adaptive and species radiation in Pachycladon is investigated through whole-genome CCP analysis.ResultsThe four analyzed Pachycladon species possess an identical karyotype structure. The consensual ancestral karyotype is most likely common to all Pachycladon species and corroborates the monophyletic origin of the genus evidenced by previous phylogenetic analyses. The ancestral Pachycladon karyotype (n = 10) originated through an allopolyploidization event between two genomes structurally resembling the Ancestral Crucifer Karyotype (ACK, n = 8). The primary allopolyploid (apparently with n = 16) has undergone genome reshuffling by descending dysploidy toward n = 10. Chromosome "fusions" were mediated by inversions, translocations and centromere inactivation/loss. Pachycladon chromosome 3 (PC3) resulted from insertional fusion, described in grasses. The allopolyploid ancestor originated in Australia, from the same or closely related ACK-like parental species as the Australian Camelineae allopolyploids. However, the two whole-genome duplication (WGD) events were independent, with the Pachycladon WGD being significantly younger. The long-distance dispersal of the diploidized Pachycladon ancestor to New Zealand was followed by the Pleistocene species radiation in alpine habitats and characterized by karyotypic stasis.ConclusionsKaryotypic stasis in Pachycladon suggests that the insular species radiation in this genus proceeded through homoploid divergence rather than through species-specific gross chromosomal repatterning. The ancestral Pachycladon genome originated in Australia through an allopolyploidization event involving two closely related parental genomes, and spread to New Zealand by a long-distance dispersal. We argue that the chromosome number decrease mediated by inter-genomic reshuffling (diploidization) could provide the Pachycladon allopolyploid founder with an adaptive advantage to colonize montane/alpine habitats. The ancestral Pachycladon karyotype remained stable during the Pleistocene adaptive radiation into ten different species.

Transcript and protein profiling identify candidate gene sets of potential adaptive significance in New Zealand Pachycladon

BackgroundTranscript profiling of closely related species provides a means for identifying genes potentially important in species diversification. However, the predictive value of transcript profiling for inferring downstream-physiological processes has been unclear. In the present study we use shotgun proteomics to validate inferences from microarray studies regarding physiological differences in three Pachycladon species. We compare transcript and protein profiling and evaluate their predictive value for inferring glucosinolate chemotypes characteristic of these species.ResultsEvidence from heterologous microarrays and shotgun proteomics revealed differential expression of genes involved in glucosinolate hydrolysis (myrosinase-associated proteins) and biosynthesis (methylthioalkylmalate isomerase and dehydrogenase), the interconversion of carbon dioxide and bicarbonate (carbonic anhydrases), water use efficiency (ascorbate peroxidase, 2 cys peroxiredoxin, 20 kDa chloroplastic chaperonin, mitochondrial succinyl CoA ligase) and others (glutathione-S-transferase, serine racemase, vegetative storage proteins, genes related to translation and photosynthesis). Differences in glucosinolate hydrolysis products were directly confirmed. Overall, prediction of protein abundances from transcript profiles was stronger than prediction of transcript abundance from protein profiles. Protein profiles also proved to be more accurate predictors of glucosinolate profiles than transcript profiles. The similarity of species profiles for both transcripts and proteins reflected previously inferred phylogenetic relationships while glucosinolate chemotypes did not.ConclusionsWe have used transcript and protein profiling to predict physiological processes that evolved differently during diversification of three Pachycladon species. This approach has also identified candidate genes potentially important in adaptation, which are now the focus of ongoing study. Our results indicate that protein profiling provides a valuable tool for validating transcript profiles in studies of adaptive divergence.

First record ofTurnip mosaic virusinPachycladonspp. (Brassicaceae): an endangered native plant species in New Zealand

Pachycladon comprises nine native species from the South Island, New Zealand, including several threatened species. Virus-like disease symptoms were observed in a Pachycladon species collection. This is the first record of a virus in Pachycladon and a further report of Turnip mosaic virus (TuMV) infections in New Zealand native hosts. This may be the first record of association between the aphid species Aulacorthum solani and Pachycladon.

A new species of Pachycladon (Brassicaceae) from limestone in eastern Marlborough, New Zealand

Pachycladon fasciarium is described as a new species from the Chalk Range, Mead Hill, and Ben More in eastern Marlborough, New Zealand. It is an obligate caldcole, occurring only on Amuri Limestone at an altitudinal range of 900–1170m. This new species is most similar to P. fastigiatum, from which it is distinguished by glabrous, narrowly linear, shorter, usually entire leaves, with a length‐to‐width ratio of 7.4–18.4:1 (av. 11.3:1). In contrast, the leaves of P. fastigiatum are either glabrous or hairy, elliptic to lanceolate, prominently serrated, and with a length‐to‐width ratio of 3.2–8.8:1 (av. 5.2:1 ). In a principal components analysis of six leaf morphological characters, plants of P. fasciarium form a group that is distinct from a second group that includes plants of P. fastigiatum sampled from throughout its distributional range. Pachycladon fasciarium is currently known from fewer than 50 plants, and its conservation status is assessed as Nationally Critical.

A Pleistocene inter-tribal allopolyploidization event precedes the species radiation of Pachycladon (Brassicaceae) in New Zealand

The Southern Alps in New Zealand contain many herbaceous plant groups that have radiated during the Plicoene–Pleistocene. The species in these genera tend to be polyploid relative to their overseas close relatives, an observation of much interest given that hybridization and allopolyploidy have recently been suggested as a possible stimulus for adaptive radiation. We were interested to determine whether or not allopollyploidy was a feature of Pachycladon, a genus which is hypothesised to have adaptively diversified onto different geological substrates in the mountains of the South Island of New Zealand. Phylogenetic analyses of five single-copy nuclear genes show that Pachycladon species have two copies of each gene representing two highly diverged evolutionary lineages from the Brassicaceae. Molecular clock analyses of all loci suggest that the two genome copies in Pachycladon diverged 8 million years ago, and that the allopolyploid origin of the genus occurred during the Pleistocene between 1.6 and 0.8 million years ago. This hybridization event at the origin of the Pachycladon radiation is perhaps the most extreme example yet reported of successful hybridization between distantly related parents.

Transcriptional and biochemical signatures of divergence in natural populations of two species of New Zealand alpinePachycladon

New Zealand is diverse in alpine and subalpine environments, a consequence of Late Tertiary tectonic and climatic change. However, few studies have sought to evaluate the importance of these environments as abiotic drivers in the diversification of plant species. Of particular interest is the Late Tertiary radiation of Pachycladon, an endemic New Zealand genus of alpine cress. Here we report observations on genome-wide levels of differential expression measured in the habitats of two closely related species of Pachycladon with distinct altitudinal preferences. Using Arabidopsis microarrays, we have identified 310 predominantly hormone- and stress-response genes up-regulated in Pachycladon fastigiata and 324 genes up-regulated in Pachycladon enysii. Expression patterns for glucosinolate biosynthesis and hydrolysis genes (MAM1, MAM-I, MAM-D, AOP2, ESP, ESM1) as well as flavonoid biosynthesis genes (F3'H, FLS, FAH1) were found to be species specific. Predicted differences in flavonoid contents were partly confirmed by high performance liquid chromatography analysis. Differences in glucosinolate profiles and glucosinolate hydrolysis products obtained by high performance liquid chromatography and gas chromatography-mass spectrometry analysis, respectively, also supported inferences from expression analyses. Five glucosinolate chemotypes were matched to known Arabidopsis ecotypes, and the potential adaptive significance of these chemotypes has been discussed. Our findings, in contrast to expectations for evolution of the New Zealand flora, suggest that biotic drivers, such as plant-herbivore interactions, are likely to be as important as abiotic drivers in the diversification of Pachycladon.

An artificial intergeneric hybrid derived from sexual hybridization between the distantly relatedArabidopsis thalianaandPachycladon cheesemanii(Brassicaceae)

An artificial intergeneric hybrid derived from sexual hybridization between Arabidopsis thaliana (female) and Pachycladon cheesemanii (male) is characterized and formally named as ×Pachydopsis hortorum. A hybrid origin for ×Pachydopsis is supported by a chromosome number of 2n = 15, with this comprising haploid gametes from A. thaliana (n = 5) and P. cheesemanii (n = 10). DNA fingerprinting using amplified fragment length polymorphism data also confirmed the parentage of the hybrid, as it comprised the additive total of all of the fragments scored for P. cheesemanii and most of those scored for A. thaliana. ×Pachydopsis is a robust perennial herb with numerous leafy rosettes and long inflorescences like P. cheesemanii, and floral parts that are intermediate between the parents. ×Pachydopsis produces sterile pollen. Female function of ×Pachydopsis is partially normal, with the pollen of A. thaliana and several species of Pachycladon readily germinating on the stigma, and pollen tubes growing down the style and transmission tissue into the ovary. No seed was formed from these pollinations, although one embryo developed but subsequently aborted at the torpedo stage. Colchicine was used to induce polyploid inflorescences, and these were confirmed by the size and density of the stomata and pollen diameter. Flowers from the polyploid inflorescences of ×Pachydopsis were backcrossed with A. thaliana Landsberg erecta and 11 seeds were produced. Four backcross hybrid plants were raised and two of these had a chromosome number of 2n = 20, consistent with a haploid gamete (n = 15) from polyploid × Pachydopsis being fertilized by a haploid pollen gamete (n = 5) from A. thaliana Landsberg erecta. Polyploid × Pachydopsis flowers backcrossed with P. cheesemanii and crossed with P. novae-zelandiae did not produce any seeds. The partially fertile intergeneric hybrid described here between Arabidopsis and Pachycladon is remarkable in that, although the two genera occur in an Arabidopsoid clade, they are separated by a reasonable phylogenetic distance. This relationship suggests that Pachycladon and ×Pachydopsis have the potential to be employed in the study of evolutionary and ecological processes using the wide array of Arabidopsis resources. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 157, 533–544.

Phylogeny, biogeography and adaptive radiation of Pachycladon (Brassicaceae) in the mountains of South Island, New Zealand

AbstractAim To report analyses and propose hypotheses of adaptive radiation that explain distributional patterns of the alpine genus Pachycladon Hook.f. – a morphologically diverse genus from New Zealand closely related to Arabidopsis thaliana.Location South Island, New Zealand.Methods Morphological and nrDNA ITS sequence phylogenies were generated for Pachycladon. An analysis is presented of species distributional patterns and attributes.Results Phylogenetic analyses of morphological characters and nrDNA ITS sequence data were found to be congruent in supporting three New Zealand clades for Pachycladon. Monophyletic groups identified within the genus are geographically distinct and are associated with different geological parent materials. Distribution maps, latitude and altitude range, and data on geological parent material are presented for the nine named and one unnamed species of Pachycladon from New Zealand.Main conclusions (a) Panbiogeographic hypotheses accounting for the origin and present‐day distribution of Pachycladon in New Zealand are not supported. (b) Species diversity and distributions of Pachycladon are explained by a Late Tertiary–Quaternary adaptive radiation associated with increasing specialization to geological substrates. Pachycladon cheesemanii Heenan & A.D.Mitch. is morphologically similar to the closest overseas relatives. It is a geological generalist and has wide latitudinal and altitudinal ranges, and we suggest it resembles the ancestral form of the genus in New Zealand. Pachycladon novae‐zelandiae (Hook.f.) Hook.f. and P. wallii (Carse) Heenan & A.D.Mitch. are a southern South Island group that predominantly occurs on Haast Schist, are polycarpic, have lobed leaves, and lateral inflorescences. Pachycladon enysii (Cheeseman) Heenan & A.D.Mitch., P. fastigiata (Hook.f.) Heenan & A.D.Mitch., and P. stellata (Allan) Heenan & A.D.Mitch. are restricted to greywacke in the eastern South Island, and are facultatively monocarpic, have serrate leaves, and stout terminal inflorescenes. (c) Present distributions of Pachycladon species may relate to Pleistocene climate change. Pachycladon enysii reaches the highest altitude of New Zealand species of Pachycladon and is most common in the Southern Alps in Canterbury. We propose that this species survived on nunataks at the height of the last glaciation. In contrast, P. fastigiata grows at a lower altitude and is absent from the high mountains of the Southern Alps. We suggest it was extirpated from this area during the last glaciation.

Embryology of two threatened species of Pachycladon (Brassicaceae)

Ovule development and embryogenesis are described for Pachycladon exilis and P. cheesemanii using thin sectioning of resin‐embedded tissue. The structures of megasporogenesis and embryogenesis in these two species of Pachycladon were similar to those described for Arabidopsis thaliana. There was, however, a significant difference in the timing of events, with an extended delay apparent in Pachycladon between anther dehiscence and the first cell divisions of the embryo and endosperm. Initial endosperm development in the two Pachycladon species was of the free nuclear type, followed by cellularisation in a manner similar to that seen in Arabidopsis. Within one day of anthesis a chalazal proliferating tissue mass developed adjacent to the chalazal pole of the embryo sac. However, free nuclear division of endosperm nuclei was not observed until 7 days after anther dehiscence. The stages of embryogenesis for the two Pachycladon species were approximately correspondent in time, but the final silique length and rate of silique elongation of P. cheesemanii was greater than for P. exilis. The siliques of P. cheesemanii also took at least an extra 7 days to desiccate at maturity.