Rising anthropogenic CO 2 in the atmosphere is accompanied by an increase in oceanic CO 2 and a concomitant decline in seawater pH (ref. 1). This phenomenon, known as ocean acidification (OA), has been experimentally shown to impact the biology and ecology of numerous animals and plants 2 , most notably those that precipitate calcium carbonate skeletons, such as reef-building corals 3 . Volcanically acidified water at Maug, Commonwealth of the Northern Mariana Islands (CNMI) is equivalent to near-future predictions for what coral reef ecosystems will experience worldwide due to OA. We provide the first chemical and ecological assessment of this unique site and show that acidification-related stress significantly influences the abundance and diversity of coral reef taxa, leading to the often-predicted shift from a coral to an algae-dominated state 4,5 . This study provides field evidence that acidification can lead to macroalgae dominance on reefs.Coral reefs contain the highest concentration of biodiversity in the marine realm, with abundant flora and fauna that form the backbone of complex and dynamic ecosystems 6 . From an anthropocentric standpoint, coral reefs provide valuable goods and services, supporting fisheries and tourism, and protect shorelines from storms 7 . Recently, widespread coral mortality has led to the flattening of reef frameworks and the loss of essential habitat 4 . This trend will be accelerated by ocean acidification (OA), as calcification is impaired, and dissolution is accelerated 8,9 . Furthermore, experimental evidence suggests that OA could enhance the growth 10 and competitive ability of fleshy macroalgae 11 . This OA-induced shift in the competitive balance between corals and algae could exacerbate direct effects of OA on calcifying reef species 12 and lead to ecosystem shifts favouring non-reef-forming algae over coral 4,5 . Understanding the individual responses of taxa to OA, as well as alteration of multi-species assemblages, is therefore critical to predicting ecosystem persistence and managing reef health in an era of global change.At present, much of what is known concerning the impacts of OA on coral reef biota has been laboratory-based experimental work focused on the responses of select taxa 2 . This has been expanded to mesocosm-based studies, allowing manipulation of groups of organisms and investigation of community responses 13 .Although these multi-species experimental studies are vital, they cannot recreate the variability (physical, chemical, biological) of real-world reef systems 14 . In an effort to overcome the limitations of laboratory studies, real-world low-saturation-state (Ω) sites have been investigated. In the eastern Pacific, nutrient and CO 2 -enriched upwelled waters impact coral calcification and the precipitation of carbonate cements, influencing the distribution of reefs 15 . In Mexico, freshwater springs depress Ω, influencing coral calcification and species distributions 16 . In Palau, restricted circulation and biological activity contribute to ...

Abstract. Theoretical models indicate that the evolution of tetraploids in diploid populations will depend on both the relative fitness of the tetraploid and that of the diploid-tetraploid hybrids. Hybrids are believed to have lower fitness due to imbalances in either the ploidy (endosperm imbalance) or the ratio of maternal to paternal genomes in their endosperm (genomic imprinting). In this study we created diploids, tetraploids, and hybrid triploids of Chamerion angustifolium from crosses between field-collected diploid and tetraploid plants and evaluated them at six life stages in a greenhouse comparison. Diploid offspring (from 2x ϫ 2x crosses) had significantly higher seed production and lower biomass than tetraploid offspring (from 4x ϫ 4x crosses). Relative to the diploid, the cumulative fitness of tetraploids was 0.67. In general, triploids (from 2x ϫ 4x, 4x ϫ 2x crosses) had significantly lower seed production, lower pollen viability, and higher biomass than diploid individuals. Triploid offspring derived from diploid maternal parents had lower germination rates, but higher pollen production than those with tetraploid mothers. Relative to diploids, the cumulative fitness of 2x ϫ 4x triploids and 4x ϫ 2x triploids was 0.12 and 0.06, respectively, providing some support for effect of differing maternal:paternal ratios and endosperm development as a mechanism of hybrid inviability. Collectively, the data show that tetraploids exhibit an inherent fitness disadvantage, although the partial viability and fertility of triploids may help to reduce the barrier to tetraploid establishment in sympatric populations.

The geographical distributions of diploid and polyploid Galax urceolata overlap in the Blue Ridge Mountains, USA. As part of an investigation into the evolutionary forces governing the establishment of polyploids and their coexistence with diploids, we examined the population frequencies of diploids, triploids and tetraploids in the area of overlap. Ploidy was inferred from estimates of DNA content, using¯ow cytometry, for 1570 individuals sampled from 42 populations. Across the entire sampling area, diploids and tetraploids were most abundant (55% and 34% of individuals, respectively), whereas triploids were least abundant (11%). Cytotype frequencies di ered signi®cantly among the northern, central and southern regions of the range (G 649.02, d.f. = 4, P < 0.0001), with diploids most frequent in the north-east and least frequent in the south-west. Twenty-six per cent of the populations contained three cytotypes, 33% contained two, and 40% had a single cytotype. Populations with two cytotypes occurred in all possible cytotype combinations, but when triploids were present, they were always in the minority. Uniform populations were either diploid (81%) or tetraploid (19%), but never triploid. Overall, populations are predominantly diploid or tetraploid but rarely evenly mixed, suggesting disruptive selection for chromosome number in G. urceolata. The contribution of ecological sorting and frequency-dependent mating success to the distribution of polyploids and diploids is discussed.Keywords: autopolyploid, distribution, DNA content,¯ow cytometry, Galax urceolata, polyploidy. IntroductionPolyploidy, or chromosome multiplication, is a central feature of many plants. Estimates vary, but between 47 and 70% of extant angiosperm species are believed to be polyploid (Masterson, 1994). Because of its broad e ects on a variety of plant attributes, polyploidy is an important mechanism of range expansion and local adaptation (Levin, 1983), and through its in¯uence on reproductive isolation is arguably one of the most important mechanisms of plant species diversi®cation (Levin, 1983). Despite its taxonomic and evolutionary signi®cance, little is known of the processes governing the origins and establishment of polyploids in natural populations (Thompson & Lumaret, 1992; Ramsey & Schemske, 1998).Geographical distributions of polyploids and their diploid progenitors provide useful insights into the population biology of polyploidy. Di erences in geographical range among cytotypes may re¯ect historical patterns of colonization or genetic di erentiation in ecological amplitude that has occurred in association, or subsequent to, polyploid formation (Levin, 1983). Where cytotype ranges overlap, local distributions o er opportunities to examine the interactions between cytotypes and to infer the evolutionary forces in¯uencing the establishment of polyploids and coexistence with diploids in natural populations (Lewis & Suda, 1976). However, relatively few studies have described the frequencies of diploid and polyploid cytotypes and...

Speciation requires the evolution of barriers to gene exchange between descendant and progenitor populations. Cryptic reproductive barriers in plants arise after pollination but before fertilization as a result of pollen competition and interactions between male gametophytes and female reproductive tissues. We tested for such gametic isolation between the polyploid Chamerion angustifolium and its diploid progenitor by conducting single (diploid or tetraploid) and mixed ploidy (1 : 1 diploid and tetraploid) pollinations on both cytotypes and inferring siring success from paternity analysis and pollen-tube counts. In mixed pollinations, polyploids sired most (79%) of their own seeds as well as those of diploids (61%) (correcting for triploid block, siring success was 70% and 83%, respectively). In single donor pollinations, pollen tubes from tetraploids were more numerous than those from diploids at four different positions in each style and for both diploid and tetraploid pollen recipients. The lack of a pollen donor´recipient interaction indicates that the tetraploid siring advantage is a result of pollen competition rather than pollenpistil interactions. Such unilateral pollen precedence results in an asymmetrical pattern of isolation, with tetraploids experiencing less gene ow than diploids. It also enhances tetraploid establishment in sympatric populations, by maximizing tetraploid success and simultaneously diminishing that of diploids through the production of inviable triploid offspring.

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