Fitness differences among diploids, tetraploids, and their triploid progeny in Chamerion angustifolium: mechanisms of inviability and implications for polyploid evolution

Can we stop transgenes from taking a walk on the wild side?

Whether the potential costs associated with broad-scale use of genetically modified organisms (GMOs) outweigh possible benefits is highly contentious, including within the scientific community. Even among those generally in favour of commercialization of GM crops, there is nonetheless broad recognition that transgene escape into the wild should be minimized. But is it possible to achieve containment of engineered genetic elements in the context of large scale agricultural production? In a previous study, Warwick et al. (2003) documented transgene escape via gene flow from herbicide resistant (HR) canola (Brassica napus) into neighbouring weedy B. rapa populations (Fig. 1) in two agricultural fields in Quebec, Canada. In a follow-up study in this issue of Molecular Ecology, Warwick et al. (2008) show that the transgene has persisted and spread within the weedy population in the absence of selection for herbicide resistance. Certainly a trait like herbicide resistance is expected to spread when selected through the use of the herbicide, despite potentially negative epistatic effects on fitness. However, Warwick et al.'s findings suggest that direct selection favouring the transgene is not required for its persistence. So is there any hope of preventing transgene escape into the wild?

Inbreeding depression and outbreeding depression in Digitalis purpurea: optimal outcrossing distance in a tetraploid

An optimal crossing distance exists within plant populations if inbreeding and outbreeding depression operate simultaneously. In a population of tetraploid Digitalis purpurea, maternal plants were pollinated with donors at four distances: 0 (self-pollination), 1, 6 and 30 m. Lifetime fitness of F1 progeny was investigated in greenhouse experiments, and significant inbreeding and outbreeding depression were detected at five vs. three life history traits. Inbreeding depression increased at later life stages, whereas outbreeding depression was relatively constant. The existence of within-population outbreeding depression suggests substantial genetic structuring at moderate distances in D. purpurea, and corroborates recent findings of significant outbreeding depression in F1 progeny in polyploids. The moderate inbreeding depression found in this predominately outcrossing population supports the notion that effects of inbreeding are less severe in polyploids than in diploids.

Population dynamics of diploid and hexaploid populations of a perennial herb

BACKGROUND AND AIMS

Despite the recent enormous increase in the number of studies on polyploid species, no studies to date have explored the population dynamics of these taxa. It is thus not known whether the commonly reported differences in single life-history traits between taxa of different ploidy levels result in differences in population dynamics.

METHODS

This study explores differences in single life-history traits and in the complete life cycle between populations of different ploidy levels and compares these differences with differences observed between different habitat types and years. Diploid and hexaploid populations of a perennial herb, Aster amellus, are used as the study system. Transition matrix models were used to describe the dynamics of the populations, and population growth rates, elasticity values and life-table response experiments were used to compare the dynamics between populations and years.

KEY RESULTS

The results indicate that between-year variation in population dynamics is much larger than variation between different ploidy levels and different habitat conditions. Significant differences exist, however, in the structure of the transition matrices, indicating that the dynamics of the different ploidy levels are different. Strong differences in probability of extinction of local populations were also found, with hexaploid populations having higher probability than diploid populations, indicating strong potential differences in persistence of these populations.

CONCLUSIONS

This is the first study on complete population dynamics of plants of different ploidy levels. This knowledge will help to understand the ability of new ploidy levels to spread into new areas and persist there, and the interactions of different ploidy levels in secondary contact zones. This knowledge will also contribute to understanding of interactions of different ploidy levels with other plant species or other interacting organisms such as pollinators or herbivores.

Sympatric diploid and hexaploid cytotypes of Senecio carniolicus (Asteraceae) in the Eastern Alps are separated along an altitudinal gradient

We explored the fine-scale distribution of cytotypes of the mountain plant Senecio carniolicus along an altitudinal transect in the Eastern Alps. Cytotypes showed a statistically significant altitudinal segregation with diploids exclusively found in the upper part of the transect, whereas diploids and hexaploids co-occurred in the lower range. Analysis of accompanying plant assemblages revealed significant differences between cytotypes along the entire transect but not within the lower part only, where both cytotypes co-occur. This suggests the presence of ecological differentiation between cytotypes with the diploid possessing the broader ecological niche. No tetraploids were detected, indicating the presence of strong crossing barriers.

Inbreeding depression in an autotetraploid herb: a three cohort field study

Autotetraploids are predicted to have reduced inbreeding depression relative to diploids. However, recent theory and information on genomic changes following autopolyploidy suggest that inbreeding depression may be closer to diploids. In three consecutive years, self and outcross pollinations were conducted on autotetraploid Campanulastrum americanum, seeds were planted into native sites, and biennial offspring were followed through seed production. Inbred individuals had lower germination rates, reduced survival, were smaller, and flowered later, producing fewer fruits with fewer seeds. Inbred offspring had 6% of the cumulative fitness of outcross offspring. Although performance varied substantially among cohorts, inbreeding depression for cumulative fitness was relatively constant, with delta ranging only from 0.92 to 0.95. C. americanum, like many outcrossing species, expressed very high amounts of inbreeding depression. This supports the hypothesis that inbreeding depression of some autotetraploids may be similar to that of diploids. Furthermore, few studies have measured temporal variation in inbreeding depression. Constant inbreeding depression given a sixfold range in cohort performance suggests that inbreeding depression may be relatively robust to environmental variation experienced by natural populations.

Pollinator-mediated assortative mating in mixed ploidy populations of Chamerion angustifolium (Onagraceae)

Establishment of polyploid individuals within diploid populations is theoretically unlikely unless polyploids are reproductively isolated, pre-zygotically, through assortative pollination. Here, we quantify the contribution of pollinator diversity and foraging behaviour to assortative pollen deposition in three mixed-ploidy populations of Chamerion angustifolium (Onagraceae). Diploids and tetraploids were not differentiated with respect to composition of insect visitors. However, foraging patterns of the three most common insect visitors (all bees) reinforced assortative pollination. Bees visited tetraploids disproportionately often and exhibited higher constancy on tetraploids in all three populations. In total, 73% of all bee flights were between flowers of the same ploidy (2x-2x, 4x-4x); 58% of all flights to diploids and 83% to tetraploids originated from diploid and tetraploid plants, respectively. Patterns of pollen deposition on stigmas mirrored pollinator foraging behaviour; 73% of all pollen on stigmas (70 and 75% of pollen on diploid and tetraploid stigmas, respectively) came from within-ploidy pollinations. These results indicate that pollinators contribute to high rates of pre-zygotic reproductive isolation. If patterns of fertilization track pollen deposition, pollinator-plant interactions may help explain the persistence and spread of tetraploids in mixed-ploidy populations.

Recombination and loss of complementation: a more than two-fold cost for parthenogenesis

Certain types of asexual reproduction lead to loss of complementation, that is unmasking of recessive deleterious alleles. A theoretical measure of this loss is calculated for apomixis, automixis and endomitosis in the cases of diploidy and polyploidy. The effect of the consequent unmasking of deleterious recessive mutations on fitness is also calculated. Results show that, depending on the number of lethal equivalents and on the frequency of recombination, the cost produced by loss of complementation after few generations of asexual reproduction may be greater than the two-fold cost of meiosis. Maintaining complementation may, therefore, provide a general short-term advantage for sexual reproduction. Apomixis can replace sexual reproduction under a wide range of parameters only if it is associated with triploidy or tetraploidy, which is consistent with our knowledge of the distribution of apomixis.

Cytotype segregation on regional and microgeographic scales in snow buttercups (Ranunculus adoneus: Ranunculaceae)

Polyploid speciation is an important source of angiosperm diversity. Insights into the origin and establishment of new polyploid species may be gained by studying the distributions of ancestral and derivative cytotypes at multiple spatial scales. Diploid (2n = 16) and tetraploid (2n = 32) snow buttercups (Ranunculus adoneus: Ranunculaceae) occur in the alpine of the central and southern Rocky Mountains. Root-tip squashes and flow cytometry were used to determine the ploidy of 1618 individuals from 35 populations. Samples from 31 of the 35 sites were entirely of one cytotype, either diploid or tetraploid. Diploid and tetraploid snow buttercups have nonoverlapping regional distributions. Where both cytotypes occur on the same site, the two are spatially segregated despite no apparent change in habitat. Triploid snow buttercups were only found at a diploid/tetraploid contact zone, while two hexaploid plants were found in tetraploid populations. Tetraploid establishment once or twice in the history of the species complex could account for the regional distribution of the two cytotypes. Habitat differentiation between cytotypes or reproductive exclusion of minority cytotypes may explain the observed segregation at both microgeographic and regional scales.

Population models of sperm-dependent parthenogenesis

Organisms that reproduce by sperm-dependent parthenogenesis are asexual clones that require sperm of a sexual host to initiate egg production, without the genome of the sperm contributing genetic information to the zygote. Although sperm-dependent parthenogenesis has some of the disadvantages of sex (requiring a mate) without the counterbalancing advantages (mixing of parental genotypes), it appears amongst a wide variety of species. We develop initial models for the density-dependent dynamics of animal populations with sperm-dependent parthenogenesis (pseudogamy or gynogenesis), based on the known biology of the common Enchytraeid worm Lumbricillus lineatus. Its sperm-dependent parthenogenetic populations are reproductive parasites of the hermaphrodite sexual form. Our logistic models reveal two alternative requirements for coexistence at density-dependent equilibria: (i) If the two forms differ in competitive ability, the form with the lower intrinsic birth rate must be compensated by a more than proportionately lower competitive impact from the other, relative to intraspecific competition, (ii) If the two forms differ in their intrinsic capacity to exploit resources, the sperm-dependent parthenogen must be superior in this respect and must have a lower intrinsic birth rate. In general for crowded environments we expect a sperm-dependent parthenogen to compete strongly for limiting resources with the sexual sibling species. Its competitive impact is likely to be weakened by its genetic uniformity, however, and this may suffice to cancel any advantage of higher intrinsic growth rate obtained from reproductive investment only in egg production. We discuss likely thresholds of coexistence for other sperm-dependent parthenogens. The fish Poeciliopsis monacha-lucida likewise obtains an intrinsic growth advantage from reduced investment in male gametes, and so its persistence is likely to depend on it being a poor competitor. The planarian Schmidtea polychroa obtains no such intrinsic benefit because it produces fertile sperm, and its persistence may depend on superior resource exploitation.

Triploid Bridge and Role of Parthenogenesis in the Evolution of Autopolyploidy

Autopolyploidization is considered to play an important role in plant evolution. In polyploidization, the polyploid evolves from the original diploid cytotype, in which the triploid state is considered to mediate the process (triploid bridge). Nevertheless, the fitness of triploid individuals seems to be too low to facilitate the polyploidization process (triploid block). The evolutionary condition of autopolyploidy was analyzed using a mathematical model focusing on the role of parthenogenesis in triploid and tetraploid individuals. In addition, offspring were assumed to arise by sexual reproduction by conjugations between haploid, diploid, and triploid gametes produced by diploid, tetraploid, and triploid individuals. According to the analysis, even if triploid block suppresses the fitness of sexually produced triploids, the polyploidization process can proceed when parthenogenesis occurs frequently. If only triploids frequently reproduce parthenogenetically, the evolutionary consequences tend to depend on the fitness of the tetraploid individuals. On the basis of a predetermined parameter set, if tetraploid fitness is relatively low, all three ploidies can coexist. Otherwise, tetraploidization occurs. In this case, triploid parthenogenesis promotes not only triploidization but also tetraploidization. However, if both triploids and tetraploids frequently reproduce parthenogenetically, the ploidy levels with the highest fitness are likely to dominate in the population through direct competition among cytotypes.

Modelling the establishment and spread of autotetraploid plants in a spatially heterogeneous environment

The establishment and spread of autotetraploids from an original diploid population in a heterogeneous environment were studied using a stochastic simulation model. Specifically, we investigated the effects of heterogeneous habitats and nonrandom pollen/seed dispersal on the critical value (micro) of unreduced 2n gamete production necessary for the establishment of autotetraploids as predicted by deterministic models. Introduction of a heterogeneous environment with random pollen/seed dispersal had little effect on the micro value. In contrast, incorporating nonrandom pollen/seed dispersal into a homogeneous environment considerably reduced the micro value. Incorporating both heterogeneous habitats and nonrandom pollen/seed dispersal may lead either to an increase or to a decrease in the micro value compared to that with random dispersal, indicating that the two factors interact in a complex way.

Diploid and polyploid cytotype distribution in Melampodium cinereum and M. leucanthum (Asteraceae, Heliantheae)

Previous chromosomal studies within Melampodium (Asteraceae, Heliantheae) of Mexico and Central America have documented chromosome numbers n = 9, 10, 11, 12, 18, 20, 23, 25 ± 1, 27, 30, and 33. Some species also have been shown to exhibit infra- and interpopulational polyploidy. The presence of cytotype mixtures is especially pronounced in the white-rayed complex, which occurs in the southwestern United States and adjacent Mexico. This group includes M. cinereum (n = 10 and 20), M. leucanthum (n = 10 and 20), and M. argophyllum (n = 30). Cytotype distribution has been newly analyzed in 415 plants from 152 populations and added to existing data from 185 plants from 113 populations, yielding information from a total of 600 individuals from 265 populations. Within M. cinereum and M. leucanthum are parapatric distributions of cytotypes, with tetraploids centered in the eastern and diploids in the western portions of their ranges. Tetraploids are most likely of autopolyploid origin, forming recurrently, with adaptations that allow colonization and establishment in new ecological regions. Contact zones are relatively narrow and only two triploid individuals have been detected. The tetraploid cytotypes probably extended eastward into central and southern Texas to the natural barriers at the edge of the Edward's Plateau in M. leucanthum and the low sandy plains in M. cinereum. The hexaploid M. argophyllum is interpreted as a relict surviving in the low mountains of northern Mexico; it may be an allopolyploid of hybrid origin between ancestors of the evolutionary lines that eventually yielded M. cinereum and M. leucanthum.

Reproductive isolation between autotetraploids and their diploid progenitors in fireweed, Chamerion angustifolium (Onagraceae)

Polyploidy is viewed as an important mechanism of sympatric speciation, but few studies have documented the reproductive barriers between polyploids and their diploid progenitors or explored the significance of assortative mating for polyploid establishment. Here we synthesize new and existing data on five prezygotic (geographic isolation, flowering asynchrony, pollinator fidelity, self-pollination, gametic selection) and two postzygotic (selection against triploid hybrids, inbreeding depression) reproductive barriers between diploid and autotetraploid individuals of the perennial plant Chamerion angustifolium. We also present estimates of realized rates of between-ploidy mating and examine the impact of assortative mating on polyploid dynamics using computer simulation. Reproductive isolation (measured from 0 to 1) was enforced by each barrier, including: geographic separation (RI = 0.41), flowering asynchrony (0.13), pollinator fidelity (0.85), self-pollination (0.44), gametic selection (0.44) and postzygotic isolation (0.87). Total reproductive isolation was 0.997, with the largest relative contributions by geography (41%) and pollinator fidelity (44%). Prezygotic barriers accounted for 97.6% isolation overall; however, tetraploids were more assortatively mating (98%) than diploids (79%). Realized reproductive isolation between ploidy levels in sympatric populations was 87% and tetraploids produced significantly fewer triploids than did diploids. Simulations indicated that the observed prezygotic isolation will reduce the strength of minority disadvantage acting on tetraploids and increase the importance of differences in viability and fertility between cytotypes in regulating polyploidy establishment.

Breaking down taxonomic barriers in polyploidy research

Polyploidy is important in the evolutionary history of plants, and it has played a crucial role in shaping the genome structures of all eukaryotes. New and rapidly improving techniques in genomics, cytogenetics and molecular ecology have resulted in a dramatic increase in publications about duplicate genes, genome rearrangements and detection of ancient duplication events. Similarly, research associated with the origins of polyploidy, its persistence in natural populations and the resulting ecological consequences is receiving more attention. Although polyploidy research has been conducted using both animal and plant systems, inferences based on cross-disciplinary comparisons have been rare. Here, I review recent developments in the field in both plants and animals, emphasizing the benefits of communication between the two groups.

Outcrossing rate and inbreeding depression in the herbaceous autotetraploid, Campanula americana

Polyploidy in angiosperms is frequently associated with an increase in self-compatibility. Self-fertilization can enhance polyploid establishment, and theory predicts reduced inbreeding depression in polyploids relative to diploids. Therefore, we may expect mating systems that promote self-fertilization or mixed-mating in polyploid species. However, few studies have measured polyploid mating systems and inbreeding depression. We report the outcrossing rate and inbreeding depression for Campanula americana, a self-compatible protandrous herb. Allozyme genotypes suggest that C. americana is an autotetraploid with tetrasomic inheritance. We found that the multilocus outcrossing rate, t(m)=0.938, did not differ from unity. This result was unexpected since previous work demonstrated that pollinators frequently move from male- to female-phase flowers on the same plant, that is, geitonogamy. Self and outcross pollinations were conducted for three populations. Offspring were germinated in controlled conditions and grown to maturity in pots in nature. Inbreeding depression was not significant for most seed and germination characters. However, all later life traits except flowering date differed between inbred and outcrossed individuals resulting in a 26% reduction in cumulative fitness for inbred plants. Limited early- and moderate later-life inbreeding depression suggest that it is buffered by the higher levels of heterozygosity found in an autotetraploid. C. americana appears to have a flexible mating system where within flower protandry and/or cryptic self-incompatibility result in a high outcrossing rate when pollinators are abundant, but self-compatibility and limited inbreeding depression maintain reproductive success when mates are limited.

Pollen competition as a unilateral reproductive barrier between sympatric diploid and tetraploid Chamerion angustifolium

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 x recipient interaction indicates that the tetraploid siring advantage is a result of pollen competition rather than pollen-pistil interactions. Such unilateral pollen precedence results in an asymmetrical pattern of isolation, with tetraploids experiencing less gene flow 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.

Fecundity and offspring ploidy in matings among diploid, triploid and tetraploid Chamerion angustifolium (Onagraceae): consequences for tetraploid establishment

Models of polyploid evolution indicate that tetraploids are more likely to establish within diploid populations when they are formed recurrently through the union of unreduced (n=2n) gametes. To account for the coexistence of diploids and tetraploids in populations of Chamerion angustifolium, diploid, triploid and tetraploid plants were crossed in all possible combinations and fecundity and ploidy using flow cytometry of the resulting progeny were measured. Combined with previous data on cytotype fitness, these data were used in a simulation to examine the impact of unreduced gametes on tetraploid evolution. Seed set per fruit was highest in 2x x 2x crosses (69%), intermediate in 4x x 4x, 2x x 4x and 3x x 4x crosses (range, 11-35%) and lowest in 3x x 2x and 3x x 3x crosses (range, 1-10%). Offspring were diploid (94%) or triploid (6%) in 2x x 2x crosses, diploid (17.5%), triploid (56%) or tetraploid (26.5%) in 3x x 2x crosses, and triploid (53%) and tetraploid (44%) in all others (4x x 4x, 3x x 4x, 2x x 4x), indicating that some gametes are unreduced, particularly in triploids. Forty-two percent of offspring, from three different crosses, had DNA contents greater than tetraploids. Computer simulations based on these results showed that unreduced gamete formation via triploids in C. angustifolium can promote the coexistence of diploids and tetraploids, but, due to law triploid fitness, is insufficient to overcome tetraploid minority disadvantage.