J. M. M. Van Damme

GYNODIOECY IN PLANTAGO-LANCEOLATA L .1. POLYMORPHISM FOR PLASMON TYPE

SummaryStudies of the morphological polymorphism for the expression of male sterility in the gynodioecious species Plantago lanceolata revealed two separate series of stamen forms from sterile to fertile. The first type of complete male sterility (MS1) shows a disturbed development of the stamens, which are strongly reduced in size. The second type (MS2) has stamens which are petaloid. In the latter type the corolla and sometimes the pistil are also affected. Evidence is presented that these differences in expression of male sterility are cytoplasmically determined. The plasmon types are designated R and P respectively. In each plasmon type a series of intermediate sex forms occurs. Field counts showed that these intermediates are a non-negligible proportion of most populations. On the average MS1 reaches higher frequencies than MS2. Twenty two out of 27 populations appeared to be polymorphic for plasmon type. The remaining five populations are probably fixed for plasmon P and their distribution over the habitats studied suggests that the fitnesses of the different sex genotypes depend on environmental conditions.SummaryStudies of the morphological polymorphism for the expression of male sterility in the gynodioecious species Plantago lanceolata revealed two separate series of stamen forms from sterile to fertile. The first type of complete male sterility (MS1) shows a disturbed development of the stamens, which are strongly reduced in size. The second type (MS2) has stamens which are petaloid. In the latter type the corolla and sometimes the pistil are also affected. Evidence is presented that these differences in expression of male sterility are cytoplasmically determined. The plasmon types are designated R and P respectively. In each plasmon type a series of intermediate sex forms occurs. Field counts showed that these intermediates are a non-negligible proportion of most populations. On the average MS1 reaches higher frequencies than MS2. Twenty two out of 27 populations appeared to be polymorphic for plasmon type. The remaining five populations are probably fixed for plasmon P and their distribution over the habitats studied suggests that the fitnesses of the different sex genotypes depend on environmental conditions.

GYNODIOECY IN PLANTAGO-LANCEOLATA L .3. SEXUAL REPRODUCTION AND THE MAINTENANCE OF MALE STERILES

SummaryThe importance of the reproductive part of the life cycle for the maintenance of the gynodioecious breeding system in Plantago lanceolata has been studied. Two male sterility types (MS1, MS2), the corresponding partial male sterility types or intermediates (IN1, IN2) and hermaphrodites (H) have been compared in four populations for seed production (ovule production [ ] female fertility) and weight per seed. MS1 plants produce more and heavier seeds than hermaphrodites. MS2 produces more ovules than H, but it is partially female sterile, so that seed production does not differ significantly between the two phenotypes. Because MS2 in addition has a lower weight per seed, it has no advantage in reproduction via seeds over any other sex type. The results for the intermediate sex types indicate that overdominance at sex determining loci may be involved in the maintenance of MS2. Reproductive differences between genotypes within the H phenotype have been found in a cultivation experiment, in accordance with theoretical models, which concerns variation in pollen production, associated with plasmon type. The mechanism of partial female sterility is discussed. The observed differences between sex types are interpreted as pleiotropic effects of the male sterility genes. It is argued that while the differences between male steriles and hermaphrodites in sexual reproduction contribute to the maintenance of gynodioecy, they probably do not explain it fully.SummaryThe importance of the reproductive part of the life cycle for the maintenance of the gynodioecious breeding system in Plantago lanceolata has been studied. Two male sterility types (MS1, MS2), the corresponding partial male sterility types or intermediates (IN1, IN2) and hermaphrodites (H) have been compared in four populations for seed production (ovule production [ ] female fertility) and weight per seed. MS1 plants produce more and heavier seeds than hermaphrodites. MS2 produces more ovules than H, but it is partially female sterile, so that seed production does not differ significantly between the two phenotypes. Because MS2 in addition has a lower weight per seed, it has no advantage in reproduction via seeds over any other sex type. The results for the intermediate sex types indicate that overdominance at sex determining loci may be involved in the maintenance of MS2. Reproductive differences between genotypes within the H phenotype have been found in a cultivation experiment, in accordance with theoretical models, which concerns variation in pollen production, associated with plasmon type. The mechanism of partial female sterility is discussed. The observed differences between sex types are interpreted as pleiotropic effects of the male sterility genes. It is argued that while the differences between male steriles and hermaphrodites in sexual reproduction contribute to the maintenance of gynodioecy, they probably do not explain it fully.

The role of tetraploids in the sexual-asexual cycle in dandelions (Taraxacum).

Apomictic plants often produce pollen that can function in crosses with related sexuals. Moreover, facultative apomicts can produce some sexual offspring. In dandelions, Taraxacum, a sexual–asexual cycle between diploid sexuals and triploid apomicts, has been described, based on experimental crosses and population genetic studies. Little is known about the actual hybridization processes in nature. We therefore studied the sexual–asexual cycle in a mixed dandelion population in the Netherlands. In this population, the frequencies of sexual diploids and triploids were 0.31 and 0.68, respectively. In addition, less than 1% tetraploids were detected. Diploids were strict sexuals, triploids were obligate apomicts, but tetraploids were most often only partly apomictic, lacking certain elements of apomixis. Tetraploid seed fertility in the field was significantly lower than that of apomictic triploids. Field-pollinated sexual diploids produced on average less than 2% polyploid offspring, implying that the effect of hybridization in the 2x–3x cycle in Taraxacum will be low. Until now, 2x–3x crosses were assumed to be the main pathway of new formation of triploid apomicts in the sexual–asexual cycle in Taraxacum. However, tetraploid pollen donors produced 28 times more triploid offspring in experimental crosses with diploid sexuals than triploid pollen donors. Rare tetraploids may therefore act as an important bridge in the formation of new triploid apomicts.

The dynamics of gynodioecy in Plantago lanceolata L. I. Frequencies of male-steriles and their cytoplasmic male sterility types

The maintenance of a gynodioecious breeding system (hermaphrodites and male-steriles) was studied in Plantago lanceolata. Cytoplasmic-nuclear inheritance is important in the maintenance of male-steriles. The male-sterile trait is cytoplasmically based (CMS), and male fertility can be restored by nuclear genes. Male-sterile frequencies differ among populations. Several hypotheses can be considered. (1) In each population a different equilibrium could be optimal because of environmentally dependent fitnesses. (2) None of the populations has reached an equilibrium (yet). (3) The populations are in different phases of a dynamic equilibrium, i.e. limit cycles. The last two explanations both predict a dynamic process, a continuous change within populations. Sex phenotype frequencies and their underlying gene frequencies were assessed. Sex phenotype frequencies changed over years within populations. We showed that these changes resulted from changes in genotype composition, which favours the two hypotheses (2 and 3) that predict a dynamic process. CMS-type frequencies were estimated using mtDNA markers. Among populations, CMS types differed in frequencies. CMSI and CMSIII seemed to exclude each other, and CMSII types occurred in all populations. In all populations, CMSI appeared to be less restored than the other CMS types. This was not expected on the basis of either of the two theoretical models that included dynamics (hypotheses 2 and 3).

Multiple CMS-restorer gene polymorphism in gynodioecious Plantago coronopus

The mode of inheritance of the male sterility trait is crucial for understanding the evolutionary dynamics of the sexual system gynodioecy, which is the co-occurrence of female and hermaphrodite plants in natural populations. Both cytoplasmic (CMS) and nuclear (restorer) genes are known to be involved. Theoretical models usually assume a limited number of CMS genes with each a single restorer gene, while reality is more complex. In this study, it is shown that in the gynodioecious species Plantago coronopus two new CMS–restorer polymorphisms exist in addition to the two that were already known, which means four CMS–restorer systems at the species level. Furthermore, three CMS types were shown to co-occur within a single population. All new CMS types showed a multilocus system for male fertility restoration, in which both recessive and dominant restorer alleles occur. Our finding of more than two co-occurring CMS–restorer systems each with multiple restorer genes raises the question how this complex of male sterility systems is maintained in natural populations.

The evolution and maintenance of gynodioecy in sexually and vegetatively reproducing plants

The evolution and maintenance of gynodioecy is explored theoretically at the ramet level in models that allow for differences between females and hermaphrodites in vegetative components of fitness as well as in sexual components.For nuclear control of male sterility, it is shown that females may be maintained by selection in an otherwise hermaphrodite population through advantages over hermaphrodites in vegetative fitness components, even if there is no sex difference in maternal fitness components. The advantage required depends on the importance of vegetative reproduction. If females produce more daughter ramets than ovule offspring, the advantage in vegetative fitness components required to maintain females is numerically smaller than the well-known requirement of a two-fold advantage in maternal fitness components.Three further findings in the nuclear models are that (a) with some parameter combinations, females attain frequencies of higher than 0·5, in which case their equilibrium frequency decreases rather than increases with increasing female advantage in seed fecundity; (b) the advantage in adult survivability required to maintain a given equilibrium frequency of females becomes smaller with increasing reliance on vegetative reproduction; and (c) the rate of approach to equilibrium is inversely proportional to the level of vegetative reproduction in the population as a whole.If male sterility is determined solely by cytoplasmic factors, females should be maintained if they have any advantage in vegetative reproduction over hermaphrodites, given that maternal fitness components are equal in the two sexes.For nuclear-cytoplasmic control, it is shown that the premise that a joint polymorphism for cytoplasmic male sterility and nuclear fertility restoration should not be maintained in the absence of differences in fitness between sex genotypes of the same sex phenotype (Charlesworth and Ganders, 1979) may be generalised to situations where sex differences in vegetative reproduction occur.The significance of vegetative reproduction in population genetic studies in general is briefly discussed.

Quantitative trait loci affecting growth-related traits in wild barley (Hordeum spontaneum) grown under different levels of nutrient supply.

The genetic basis of phenotypic plasticity of relative growth rate (RGR), its components and associated morphological traits was studied in relation to nutrient limitation. In all, 140 F3 lines from a cross, made between two Hordeum spontaneum (wild barley) accessions sampled in Israel, were subjected to growth analysis under two nutrient levels. Quantitative trait loci (QTLs) were detected for RGR and three of its components, leaf area ratio (LAR), specific leaf area and leaf mass fraction (LMF). Indications for close linkage (potential pleiotropy) were found, for example, for LAR and LMF. An interesting case is on chromosome 6, at which QTLs for RGR and seed mass were detected in the same region. These QTLs had opposite additive effects, supporting earlier results that plants growing from lighter seeds had a higher RGR. Only two QTLs were significant under both nutrient conditions, suggesting large QTL × environment interactions for most traits. For 21 out of 26 QTLs, however, the additive genetic effect was of identical sign in both nutrient environments, but reached the significance threshold in only one of them. Nevertheless, some QTLs detected in one of the two environments had virtually no effect in the other, and QTLs for plasticity were detected for RGR, LAR and LMF, as well as for some morphological traits. QTLs with opposite effects under high and low nutrients were not found. Thus, at the genetic level, there was no evidence for a trade-off between faster growth at high versus low nutrient levels.

Nuclear–Cytoplasmic male-sterility in diploid dandelions

Male-sterility was found in diploid dandelions from two widely separated populations from France, and its inheritance was analysed by crossing a diploid male-sterile dandelion to diploid sexuals and triploid apomicts. Nuclear genetic variation, found in full-sib families, segregated for male-fertility, partial male-sterility, and full male-sterility, and also segregated for small-sized versus normally sized pollen. The crossing results are best explained by a cytoplasmic male-sterility factor in combination with two dominant restorer genes. Involvement of the cytoplasmic male-sterility factor was further investigated by chloroplast haplotyping. Male-sterility was exclusively associated with a rare chloroplast haplotype (designated 16b). This haplotype was found in seven male-sterile plants and one (apparently restored) male-fertile individual but does not occur in 110 co-existing male-fertile plants and not in several hundreds of individuals previously haplotyped. Apomicts with cytoplasmic male sterility were generated in some test crosses. This raises the question as to whether the male sterility found in natural dandelion apomicts, is of cytoplasmic or of nuclear genetic nature. As many breeding systems in Taraxacum are involved in shaping population structure, it will be difficult to predict the evolutionary consequences of nuclear–cytoplasmic male-sterility for this species complex.

A restorer gene in gynodioecious plantago coronopus subject to selection in the gametophytic and seedling stage

The genetic basis of male sterility in the gynodioecious species Plantago coronopus was studied. As the aim was to find strains with a single, varying nuclear locus for male sterility, cytoplasmic variation was avoided by starting with one open pollinated female. The progeny appeared to vary for a single locus with the homozygous recessive being male sterile. The locus was designated msl. Expected segregation ratios were disturbed by two different factors, both leading to a shortage of male steriles. An accidental drought in Petri dishes caused a relatively higher mortality of seedlings with a male sterility genotype in most cases. When this effect was avoided a systematic shortage of male steriles remained in many crosses. An experiment with varying levels of pollen competition justified the interpretation that selection at the gametophytic stage was involved. Pollen carrying the sterility allele was estimated to have a fertilization success relative to pollen with the restorer allele of 0.56. Finally the role of the msl locus in the genetics of male sterility in natural populations was assessed by crossing heterozygously and homozygously restored plants with male steriles collected from the wild. It appeared that the inheritance of male sterility is more complicated than the msl locus alone, which was expected. If the selection effects on the msl locus observed in this study occur in nature, this would have interesting consequences for the evolutionary dynamics of gynodioecy.The genetic basis of male sterility in the gynodioecious species Plantago coronopus was studied. As the aim was to find strains with a single, varying nuclear locus for male sterility, cytoplasmic variation was avoided by starting with one open pollinated female. The progeny appeared to vary for a single locus with the homozygous recessive being male sterile. The locus was designated msl. Expected segregation ratios were disturbed by two different factors, both leading to a shortage of male steriles. An accidental drought in Petri dishes caused a relatively higher mortality of seedlings with a male sterility genotype in most cases. When this effect was avoided a systematic shortage of male steriles remained in many crosses. An experiment with varying levels of pollen competition justified the interpretation that selection at the gametophytic stage was involved. Pollen carrying the sterility allele was estimated to have a fertilization success relative to pollen with the restorer allele of 0.56. Finally the role of the msl locus in the genetics of male sterility in natural populations was assessed by crossing heterozygously and homozygously restored plants with male steriles collected from the wild. It appeared that the inheritance of male sterility is more complicated than the msl locus alone, which was expected. If the selection effects on the msl locus observed in this study occur in nature, this would have interesting consequences for the evolutionary dynamics of gynodioecy.

The relationship between relative growth rate and susceptibility to aphids in wild barley under different nutrient levels

The Resource Availability Hypothesis (RAH) states that plants with a low Relative Growth Rate (RGR) and high levels of defence against herbivores or pathogens are favoured in habitats with low resource availability, whereas plants with a high potential RGR and low levels of defence are favoured in environments with high resource availability. High levels of defence are expected to result in lower reproduction and/or growth of the herbivores or pathogens. To test this hypothesis, four accessions of each of nine natural Hordeum spontaneum (wild barley) populations were grown in a climate chamber under two levels of nutrient supply. Susceptibility to Schizaphis graminum (greenbug) was quantified by placing a single adult greenbug on each plant and measuring its realised fecundity after 8 days. Data on potential RGR were available from a previous experiment. No support for the RAH was found. The correlation between potential RGR and greenbug reproduction was not significant, neither at the high nor at the low level of nutrient supply. Furthermore, on average plants grown under high and low nutrients did not differ in susceptibility. However, accessions-within-populations differed in the way susceptibility was affected by nutrient supply, and most accessions had a higher susceptibility under nutrient-poor conditions. It could be that these accessions differed in the spectrum of secondary metabolites they produced. Whatever the cause, the genetic variation for the reaction in susceptibility to nutrient supply suggests that selection could act in favour of more or less plasticity in plants without any apparent change in potential RGR.