Phylogeography of Arabis serrata (Brassicaceae) in the Japanese Archipelago based on chloroplast DNA variations

Arabis serrata (Brassicaceae), a perennial plant widely distributed along the Japanese Archipelago, occurs in various habitats: for example, limestone zones, serpentine barrens, volcanic soils, and roadsides. It likely survived by adapting to its surrounding environment, resulting in great morphological and ecological variation. In this study, we performed a phylogeographic analysis to examine past changes in the distribution of A. serrata following climate oscillations during the Pleistocene. To cover most of A. serrata's range, leaves were collected from eight to ten individuals randomly selected from each of 37 populations in the Japanese Archipelago. Two chloroplast noncoding regions of the samples were amplified and sequenced: trnT(GGU)‐psbD and trnH(GUG)‐psbA spacers. Twenty‐five haplotypes were detected and distinguished by 31 substitutions. Four main haplotypes were observed in many populations distributed throughout the Japanese Archipelago. According to the genetic boundaries detected using the Monmonier algorithm, A. serrata is clustered into four groups, each including several populations: Hokkaido Island, northern mainland Honshu, central Japan, and western Japan. The boundaries, however, were not robust because all genetic parameters did not support the differentiation among groups. These results indicate the absence of an obvious geographic structure in the distribution of A. serrata, suggesting that this species has experienced a rapid range expansion in postglacial times.     

Patterns of association between crucifers and their flower-mimic pathogens: host jumps are more common than coevolution or cospeciation

Morphological and molecular phylogenies of animal parasites have often shown parallel cladogenesis, supporting hypotheses of coevolution. Few studies of the phylogenetic history for plants and their pathogens exist. Gene-for-gene interactions suggest that plant pathogens ought to have similar phylogenetic histories as their hosts. However, high dispersability combined with an inability to choose to leave if an inappropriate host has been landed on could increase the likelihood of host jumps and thus decrease phylogenetic congruence between plant pathogens and their hosts. In this study, I examined the pattern of association between the flower-mimicking crucifer rusts and their hosts by comparing independent host phylogenies (based on both cpDNA trnL-F introns and nuclear internal transcribed spacer [ITS] sequences) with that of their rust pathogens (based on ITS sequences). The expectation was that if the pathogens coevolved or cospeciated with their hosts, then their phylogenies should be congruent. Host-tracking coevolution can be differentiated from cospeciation by examining the times of divergence: If the pathogens are younger than the hosts, then it is likely that host tracking has occurred. For the crucifer rusts and their hosts, there was little evidence of parallel cladogenesis, suggesting that both cospeciation and coevolutionary tracking are rare. Instead, the most common pattern was one of host jumps to geographically associated taxa. There are at least three factors that may have contributed to the geographic structuring of the data. First, along the east-west transect stretching from the Rocky Mountains to California, large differences in rainfall and the timing of rainfall may reduce long-distance gene flow. Second, although dispersal of infectious spores is by wind, sexual reproduction of these fungi depends on insects, which move short distances. Third, host shifts are most likely to occur to geographically available taxa. Any species that grows adjacent to infected plants will be exposed to millions of spores, and the probability of eventual infection by a new mutant increases with greater exposure. Thus, patterns of association between the crucifers and their flower-mimic pathogens reflect jumps to geographically available hosts, which are not necessarily those that are most closely related.     

Arabis gemmifera is a hyperaccumulator of Cd and Zn

Hyperaccumulators are essential for phytoremediation of heavy metals. In Europe and North America, many studies have been conducted to find more effective plants for phytoremediation of various pollutants. In Japan, this field of research has just recently come more into focus. A type of fern in Japan, Athyrium yokoscense, is well known as a hyperaccumulator of Cd and Zn. However, it is not suitable for phytoremediation because it is a summer green and grows slowly. Therefore, in order to find hyperaccumulators other than from A. yokoscense, we surveyed plants growing at polluted sites in Japan. We found that the Brassicae Arabis gemmifera is a hyperaccumulator of Cd and Zn, with phytoextraction capacities almost equal to Thlaspi caerulescens.     

Identification of a <Emphasis Type="Italic">Lotus</Emphasis> viral pathogen

A virus collection was used to identify a pathogen suitable for laboratory use with the model legume Lotus japonicus. Several Lotus species or L. japonicus accessions were tested and various degrees of susceptibility to the Arabis mosaic virus derived from barley (ArMV-ba) were found. Virus multiplication and persistence in Lotus tissue were examined, as well as plant responses to it. Sensitivity to the virus among the accessions and species is discussed in light of their geographical origin. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Cost of resistance and tolerance under competition: the defense-stress benefit hypothesis

Defense costs provide a major explanation for why plants in nature have not evolved to be better defended against pathogens and herbivores; however, evidence for defense costs is often lacking. Plants defend by deploying resistance traits that reduce damage, and tolerance traits that reduce the fitness effects of damage. We first tested the defense-stress cost (DSC) hypothesis that costs of defenses increase and become important under competitive stress. In a greenhouse experiment, uniparental maternal families of the host plant Arabis perennans were grown in the presence and absence of the bunch grass Bouteloua gracilis and the herbivore Plutella xylostella. Costs of resistance and tolerance manifest as reduced growth in the absence of herbivory were significant when A. perennans grew alone, but not in the competitive environment, in contrast to the DSC hypothesis. We then tested the defense-stress benefit (DSB) hypothesis that plant defenses may benefit plants in competitive situations thereby reducing net costs. For example, chemical resistance agents and tolerance may also have functions in competitive interactions. To test the DSB hypothesis, we compared differentially competitive populations for defense costs, assuming that poorer competitors from less dense habitats were less likely to have evolved defenses that also function in competition. Without competitive benefits of defenses, poorer competitors were expected to have higher net costs of defenses under competition in accordance with DSB. Populations of A. perennans and A. drummondii that differed dramatically in competitiveness were compared for costs, and as the DSB hypothesis predicts, only the poor competitor population showed costs of resistance under competition. However, cost of tolerance under competition did not differ among populations, suggesting that the poor competitors might have evolved a general stress tolerance. Although the DSC hypothesis may explain cases where defense costs increase under stress, the DSB hypothesis may explain some cases where costs decrease under competitive stress.     

Three times out of Asia Minor: the phylogeography of Arabis alpina L. (Brassicaceae)

Arabis alpina is a characteristic plant in arctic-alpine habitats and serves as a classical example to demonstrate biology, ecology and biogeography of arctic-alpine disjuncts. It has a wider distribution than most other arctic-alpine plants, covering all European mountain systems, the Canary Islands, North Africa, the high mountains of East Africa and Ethiopia, the Arabian Peninsula and mountain ranges of Central Asia in Iran and Iraq. Additionally it is found in the northern amphi-Atlantic area including northeastern North America, Greenland, Iceland, Svalbard and northwestern Europe. We used markers from the nuclear (internal transcribed spacer of ribosomal DNA) and chloroplast genome (trnL-F region) to reconstruct its phylogeographic history. Both markers revealed clear phylogeographic structure. We suggest that A. alpina originated in Asia Minor less than 2 million years ago based on synonymous mutation rates of different genes (plastidic matK, nuclear adh and chs). From the Asian ancestral stock one group migrated via the Arabian Peninsula to the East African high mountains. A second group gave rise to all European and northern populations, and also served as source for the northwest African populations. A third group, which is still centred in Asia, migrated independently southwards and came into secondary contact with the East African lineage in Ethiopia, resulting in high genetic diversity in this area. In the Mediterranean regions, the genetic diversity was relatively high with numerous unique haplotypes, but almost without geographic structure. In contrast, the populations in the northern amphi-Atlantic area were extremely depauperate, suggesting very recent (postglacial) expansion into this vast area from the south.     

Differentiating the effects of origin and frequency in reciprocal transplant experiments used to test negative frequency-dependent selection hypotheses

Reciprocal transplant experiments have been used to estimate the probability that negative frequency-dependent selection by natural enemies has occurred in host populations by determining whether pest populations are less adapted to “foreign” (rare) hosts, which originate from a population with which the pests have not coevolved. However, these experiments usually confound the effects of frequency and origin: the rare genotypes are also genotypes that did not originate at a site. When clonal organisms are used, and the clones occur in more than one population, it is possible to separate the effects of origin and frequency. Here I present the results of an experiment in which Arabis clones of known frequency were reciprocally transplanted among sites. Contrary to expectations, clones at their site of origin had less disease, less herbivory, and higher fitness than foreign clones. However, variation within and among sites in herbivory and infection was large, suggesting that the number of sites and clones needed to thoroughly test the hypothesis of negative frequency-dependent selection in this system is very large: thus, these results are suggestive but not conclusive. Received: 20 October 1997 / Accepted: 8 February 1998     

Approximate Bayesian Computation Untangles Signatures of Contemporary and Historical Hybridization between Two Endangered Species

Contemporary gene flow, when resumed after a period of isolation, can have crucial consequences for endangered species, as it can both increase the supply of adaptive alleles and erode local adaptation. Determining the history of gene flow and thus the importance of contemporary hybridization, however, is notoriously difficult. Here, we focus on two endangered plant species, Arabis nemorensis and A. sagittata, which hybridize naturally in a sympatric population located on the banks of the Rhine. Using reduced genome sequencing, we determined the phylogeography of the two taxa but report only a unique sympatric population. Molecular variation in chloroplast DNA indicated that A. sagittata is the principal receiver of gene flow. Applying classical D-statistics and its derivatives to whole-genome data of 35 accessions, we detect gene flow not only in the sympatric population but also among allopatric populations. Using an Approximate Bayesian computation approach, we identify the model that best describes the history of gene flow between these taxa. This model shows that low levels of gene flow have persisted long after speciation. Around 10 000 years ago, gene flow stopped and a period of complete isolation began. Eventually, a hotspot of contemporary hybridization was formed in the unique sympatric population. Occasional sympatry may have helped protect these lineages from extinction in spite of their extremely low diversity.     

高山植物圆锥南芥的光合系统耐热性及其修复机制(英文)

高温胁迫包括极端高温和中高温,严重影响了植物的一系列生理活动,尤其是光合作用,而植物应对极端高温和中高温胁迫具有不同的策略。高山植物因长期生长于相对寒冷的环境中,相比而言应缺少对高温胁迫的适应机制。本文以圆锥南芥作为一种高山模式植物来探索其在中高温下是否表现出耐热能力,如果具有耐热能力,那么在光合方面与拟南芥存在怎样的差异。研究发现,圆锥南芥在中高温处理后具有更高的光化学效率及快速可逆的恢复过程,表现出了较强的耐热能力。两物种的F0没有明显的差异,而圆锥南芥在热处理后及恢复过程中具有更高的Fm,促进其快速光合修复。在热处理后,非光化学能量耗散快速瞬时上升,及时保护光系统II免受光损伤和热伤害,另外,HSP101蛋白迅速诱导可能启动了光化学修复。最后,圆锥南芥在严重高温处理后具有更高的存活率再次验证了它在中高温下的耐热能力。结果表明,圆锥南芥具有更耐热的光合系统以及有效的光合修复机制来耐受中高温胁迫。