Pollen and stigma size changes during the transition from tristyly to distyly in Oxalis alpina (Oxalidaceae)

• Pollen and stigma size have the potential to influence male fitness of hermaphroditic plants, particularly in species presenting floral polymorphisms characterised by marked differences in these traits among floral morphs. In this study, we take advantage of the evolutionary transition from tristyly to distyly experienced by Oxalis alpina (Oxalidaceae), and examined whether modifications in the ancillary traits (pollen and stigma size) respond to allometric changes in other floral traits. Also, we tested whether these modifications are in accordance with what would be expected under the hypothesis that novel competitive scenarios (as in distylous‐derived reproductive system) exert morph‐ and whorl‐specific selective pressures to match the available stigmas.
• We measure pollen and stigma size in five populations of O. alpina representing the tristyly–distyly transition.
• A general reduction in pollen and stigma size occurred along the tristyly–distyly transition, and pollen size from the two anther levels within each morph converged to a similar size that was characterised by whorl‐specific changes (increases or decreases) in pollen size of different anthers in each floral type.
• Overall, results from this study show that the evolution of distyly in this species is characterised not only by changes in sexual organ position and flower size, but also by morph‐specific changes in pollen and stigma size. This evidence supports the importance of selection on pollen and stigma size, which increase fitness of remaining morphs following the evolution of distyly, and raises questions to explore on the functional value of pollen size in heterostylous systems under pollen competition.

     

Genetic basis of tristyly in tetraploid Oxalis alpina (Oxalidaceae)

The inheritance of style‐morphs was investigated in tetraploid populations of tristylous Oxalis alpina (Oxalidaceae) to determine if alleles controlling style‐morphs are expressed at duplicated loci. In tetraploid populations, a dominant S allele leads to expression of the short‐styled phenotype at the short/non‐short locus and is epistatic to the M allele at the mid/long locus. The M allele results in expression of the mid‐styled phenotype but only if the S allele is absent. Long‐styled morphs are homozygous recessive at the short and mid loci. Test crosses of many tetraploid short‐styled individuals resulted in segregations of short‐, mid‐ and long‐styled individuals which, because of linkage between the short and mid loci, can only occur with polyploidy and expression of alleles at duplicated loci. Segregation patterns from three crosses suggest the possibility of disomic inheritance via preferential pairing of chromosomes in tetraploid populations of O. alpina. Segregation patterns in the progeny of mid‐styled individuals indicated that only a few individuals had more than one copy of the M allele, despite the potential for accumulation of M alleles via self‐fertilization of partially self‐compatible mid‐styled morphs in some populations. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 179 , 308–318.     

Different patterns of colonization of Oxalis alpina in the Sky Islands of the Sonoran desert via pollen and seed flow

Historical factors such as climatic oscillations during the Pleistocene epoch have dramatically impacted species distributions. Studies of the patterns of genetic structure in angiosperm species using molecular markers with different modes of inheritance contribute to a better understanding of potential differences in colonization and patterns of gene flow via pollen and seeds. These markers may also provide insights into the evolution of reproductive systems in plants. Oxalis alpina is a tetraploid, herbaceous species inhabiting the Sky Island region of the southwestern United States and northern Mexico. Our main objective in this study was to analyze the influence of climatic oscillations on the genetic structure of O. alpina and the impact of these oscillations on the evolutionary transition from tristylous to distylous reproductive systems. We used microsatellite markers and compared our results to a previous study using chloroplast genetic markers. The phylogeographic structure inferred by both markers was different, suggesting that intrinsic characteristics including the pollination system and seed dispersal have influenced patterns of gene flow. Microsatellites exhibited low genetic structure, showed no significant association between geographic and genetic distances, and all individual genotypes were assigned to two main groups. In contrast, chloroplast markers exhibited a strong association between geographic and genetic distance, had higher levels of genetic differentiation, and were assigned to five groups. Both types of DNA markers showed evidence of a northward expansion as a consequence of climate warming occurring in the last 10,000 years. The data from both types of markers support the hypothesis for several independent transitions from tristyly to distyly.     

Microsatellite markers isolated from polyploid wood‐sorrel Oxalis alpina (Oxalidaceae)

Twelve polymorphic microsatellite loci were isolated from polyploid alpine wood‐sorrel, Oxalis alpina (Oxalidaceae), and optimized for future studies of its breeding system. The loci were screened for variability among 72 individuals from Pinos Altos, New Mexico. The primers amplified loci with allele number ranging from two to 17 per locus and with estimates of Nei's genetic diversity varying from 0.10 to 0.99. These primers provide an opportunity to use polymorphic DNA markers to study the causes of breeding system variability in this species.     

Purging of inbreeding depression within a population of Oxalis alpina (Oxalidaceae)

? Premise of the study: Variation among individuals in levels of inbreeding depression associated with selfing levels could influence mating system evolution by purging deleterious alleles, but empirical evidence for this association is limited. ? Methods: We investigated the association of family-level inbreeding depression and presumed inbreeding history in a tristylous population of Oxalis alpina (Oxalidaceae). ? Key results: Mid-styled individuals possessed the greatest degree of self-compatibility (SC) and produced more autogamous capsules than short- or long-styled individuals. Offspring of highly self-compatible mid-styled individuals showed reduced inbreeding depression. Mid-styled plants that produced capsules autogamously exhibited reduced stigma-anther separation compared to mid-styled plants that produced no capsules autogamously. Reduced inbreeding depression was not correlated with stigma-anther separation, suggesting that self-compatibility and autogamy evolve before morphological changes in stigma-anther separation. ? Conclusions: Purging of inbreeding depression occurred in SC mid-styled maternal families. Low inbreeding depression in SC mid-styled plants may lead to retention of the mid-styled morph in populations, despite the occurrence of higher selfing rates in mid-styled relative to short- or long-styled morphs. Variation among individuals in levels of self-fertilization within populations may lead to associations between inbreeding lineages and lower levels of inbreeding depression, influencing the evolution of mating systems.     

The role of historical factors and natural selection in the evolution of breeding systems of Oxalis alpina in the Sonoran desert ‘Sky Islands’

Pleistocene climatic oscillations are known to influence the patterns of genetic diversity and the distribution of traits that are the target of selection. Here, we combine phylogeographical and ecological niche modelling (ENM) approaches to explore the influence of historical factors (Pleistocene climatic shifts) and natural selection on the evolution of distyly (two floral morphs) from tristyly (three floral morphs) of Oxalis alpina in the Sky Islands of the Sonoran Desert. Molecular data and ENM indicate that historical factors have had a strong influence on the genetic structure and the geographical distribution of reproductive systems of O. alpina. Moreover, genetic results suggest the possibility that distylous populations do not represent a monophyletic group. We propose that the combined effects of natural selection and genetic drift have influenced the tristyly–distyly transition.     

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Aberrant pollen in southern African Oxalis (Oxalidaceae)

The formation of aberrant pollen is a well‐known phenomenon reported in various angiosperm families.The presence and range of aberrant pollen grains among southern African members of Oxalis L. were studied by SEM analysis. Seven categories of grain aberrations and two successiform series were identified. Grain aberrations were most frequently found amongst pollen with supra‐areolate exine structures and always affected the number and arrangement of the colpi. The exine structure is never altered. No distinct correlation could be detected between grain aberrations and ploidy levels.     

Chromosome diversity of South American Oxalis (Oxalidaceae)

Mitotic or meiotic chromosome studies are reported for 39 species or subspecies of Oxalis from South America belonging to 14 sections. Chromosome numbers of 34 of these taxa are reported for the first time. Diploids and polyploids with six different basic chromosome numbers x=5, 6, 7, 8, 9 and 11 are described. Thirteen species of subgenus lhamnoxys were analysed and two new basic chromosome numbers were observed in diploid entities of this subgenus, x = 6 and x=9. The underground stem-bearing entities of Oxalis subgenus Oxalis studied (in sections Articulatae, Jonoxalis and Palmatifoliae) are mostly diploids and polyploids with a basic chromosome number x=7. Five species of section Carnosa are diploids with x = 9. In species of sections Rosea, Ortgieseae, Clematodes and Laxae the basic chromosome numbers x = 6, 7, 8 and 9 were observed. Groups of related species sharing the same chromosome number are discussed with the aim of improving the infrageneric delimitation of the genus. The basic chromosome number x=6 seems to be primitive in the genus and other basic chromosome numbers probably appeared several times in the course of chromosome evolution of Oxalis .     

A new species of Oxalis section Corniculatae (Oxalidaceae) from the Balearic islands

A new species, Oxalis ferae Llorens, Gil & Cardona (Oxalidaceae) from the Balearic islands (Spain) is described and illustrated. It is considered to be endemic to Mallorca. The morphological differences between the more closely related taxa of Oxalis section Corniculatae are discussed. IUCN threatened category and observations on the ecology of the populations are noted.  © 2005 The Linnean Society of London, Botanical Journal of the Linnean Society , 2005, 148 , 489–493.     

The spatio-ecological segregation of different cytotypes of Oxalis obtusa (Oxalidaceae) in contact zones

Contact zones of different cytotypes provide an opportunity to address evolutionary mechanisms underlying the origin, establishment and maintenance of karyological diversity at intraspecific level. We explored the fine-scale distribution of ploidy levels of Oxalis obtusa in seven mixed-ploidy sites in the Western and Northern Cape Provinces, South Africa, and searched for potential isolating barriers promoting assortative mating. Five different ploidy levels (2x, 3x, 4x, 6x and 8x) were detected among 336 samples collected at 112 microsites. The studied sites were inhabited by two (2 sites), three (4 sites) or even all five (1 site) different cytotypes. Despite their sympatric growth, different ploidies show some spatio-ecological segregation. The greatest differences among microsites hosting different cytotypes were found in precipitation parameters. There is a clear altitudinal gradient in ploidy composition in the most ploidy-variable site, Pakhuis Pass. Our results show that a combination of niche partitioning and clumping of same-ploidy individuals due to vegetative reproduction seems to be efficient reproductive barriers, which limit inter-ploidy gene flow in the zones of ploidy contact and contribute to the long-term maintenance of cytotype mixtures in O. obtusa.     

Population variation of leaflet sleep movements in Oxalis grandis (Oxalidaceae)

To identify intraspecific variation in the expression of circadian leaflet movements, we observed changes in leaflet elevation angle in a controlled environment and in the field. Two morphologically and ecologically distinct populations of Oxalis grandis were compared: a typical mesic forest ecotype and an atypical, densely hirsute ecotype found on partially exposed shale outcroppings. Significant genotypic variation was detected within both populations. The controlled environment experiment revealed significant differences in the intrinsic rhythm between the two populations, primarily at the beginning and end of the photophase. We observed leaflet ascent and descent prior to lights-on and lights-off, respectively, processes we define as anticipatory. Hirsute ecotype plants showed a greater anticipation of lights-on, whereas the mesic ecotype plants showed a greater anticipation of lights-off. In the field, significant differences in leaflet elevation between the populations occurred in early morning and late afternoon, but not immediately preceding dusk. Contrary to the controlled environment experiment, mesic ecotype plants showed greater anticipation of dawn; plants from both populations showed similar anticipation of dusk. Plants of the hirsute ecotype exhibited higher sensitivity to sudden fluctuations in light level. We hypothesize that differences in light sensitivity partially explain the differences in the leaflet angle patterns between the field and controlled environments.     

The evolution and age of populations of Scaphinotus petersi Roeschke on Arizona Sky Islands (Coleoptera, Carabidae, Cychrini)

Populations of the ground beetle Scaphinotus petersi are isolated in subalpine conifer forest habitats on mountain ranges or Sky Islands in southeastern Arizona. Previous work on this species has suggested these populations have been isolated since the last post-glacial maximum times as warming caused this cool adapted species to retreat to high elevations. To test this hypothesis, we inferred the phylogeny from mitochondrial DNA sequence data from several Arizona Sky Island populations of Scaphinotus petersi and estimated the divergence time of the currently isolated populations. We found two major clades of Scaphinotus petersi, an eastern clade and a western group. Our results indicated most mountain ranges form clades except the Huachucas, which are polyphyletic and the Santa Catalinas, which are paraphyletic. We estimated the Pinaleño population is much older than the last glacial maximum, but the Huachuca and Pinal populations may have been fragmented from the Santa Catalina population since the post-glacial maximum times.     

Leaf longevity of Oxalis acetosella (Oxalidaceae) in the Catskill Mountains, New York, USA

Leaf habit correlates with multiple physiological traits. Understanding ecophysiology is therefore dependent on knowledge of leaf habit. A variety of leaf habits exists within forest understory plant communities. Oxalis acetosella is one such understory plant and has long been considered a wintergreen, meaning that it keeps a set of leaves for one full year, replacing them with a new set during spring. To assess the leaf habit of O. acetosella and place it into a classification scheme of leaf habits, leaves of four populations of O. acetosella were repeatedly censused for two years in a northern hardwood forest of the Catskill Mountains, New York, USA. New leaves developed and old leaves senesced throughout the year, yielding a continual replacement of leaves and a summer peak in leaf number. Leaves that developed in the fall and winter had longer maximum life spans than leaves that developed during the summer. The name "seasonalgreen" is suggested to describe the continual development, senescence, and presence of leaves and annual peak in leaf number within O. acetosella. The functional significance of this leaf habit in this species and the possibility of its presence in other species deserve further study.     

The evolution of distyly in Primula vulgaris

Experiments on pollen flow and seed production were performed in populations of P. vulgaris in order to examine the roles of selection for reduced self-pollination in a partially self-fertile morph, selection for reduced stigma clogging, selection for a pollen saving effect, and selection for disassortative pollination in the evolution of morphological distyly (reciprocal herkogamy). Selection for reduced self-pollination and disassortative pollination were shown to have a plausible role in the evolution of this dimorphism. Selection for reduced stigma clogging and pollen saving appeared to have no obvious role in the evolution of morphological distyly.     

Expression of stylar incompatibility in the Andean clonal tuber crop oca (Oxalis tuberosa Mol., Oxalidaceae)

Long-, mid-, and short-styled clonal accessions of oca (Oxalis tuberosa) were intercrossed in a complete diallelic design. Pollen tube growth in styles was monitored in all diallelic crosses. Pollen fertility was estimated by two tests: staining of pollen grains with aceto-carmine and detection of β-galactosidase activity by the substrate X-Gal. The two methods of pollen fertility estimation were equally useful to detect fertility levels. Pollen originating from short stamens had the highest fertility (85%) and pollen from long stamens had the lowest fertility (70%). Pollen fertility was high throughout, but its degree varied with the stylar morph on which the pollen was formed. Long-styled accessions had the highest rates of fertile pollen. Differences in pollen fertility at different anther levels in the same style morph were also apparent. Pollen grain diameter of the six morph-anther level combinations was inversely correlated with pollen fertility. Pollen grains from long stamens were the largest and pollen grains from short stamens were the smallest. Neither pollen fertility nor pollen grain size had an influence on pollen tube growth in the style or on fruit and seed set. Pollen tubes growing within the styles were inhibited at a different level for each of the 18 cross combinations in the diallel. Although legitimate crosses had greatest pollen tube growth, some of the illegitimate inter- and intramorph crosses had equally high scores. Of all illegitimate crosses, mid-styled seed parents had the lowest level of stylar incompatibility. Fruit and seed set were highly correlated with the extent of pollen tube growth in the style. The number of pollen tubes entering ovules in a flower was in good agreement with the number of seeds produced per fruit. Therefore, it is concluded that stylar incompatibility is the major determinant of limited seed formation in oca even in the most successful legitimate cross combinations. Received: 1 September 1999 / Revision accepted: 4 April 2000