Functional heterostyly in Tylosema esculentum (Caesalpinioideae)

Tylosema esculentum is a long-lived perennial species endemic to arid areas of southern Africa. Its potential as a crop species has long been recognized as a result of the high oil and protein content of its seeds. The reproductive biology and breeding systems of the species were investigated in wild and experimental populations growing in Botswana. Field observations confirmed that the species is heterostylous with the pistil and anthers exhibiting reciprocal heights in the two morphs, although pollen size and sculpturing do not vary. The wet, nonpapillate stigma characteristic of the species is the first to be reported in the Caesalpinioideae. In vivo and in vitro diallel crossing experiments demonstrated that a diallelic self-incompatability system exists in T. esculentum. The major site of pollen tube inhibition in the intramorph crosses was found to be in the style. This is the first report of functional heterostyly in the Fabaceae and of a confirmed self-incompatibility system in the Caesalpinioideae. Three separate lines of evidence, the monitoring of fruit development in open-pollinated plants, fruit set in diallel crossing experiments, and observations made in wild populations, demonstrated that fruit set and, by implication, seed set, are very low in this species. Floral abscission was a major limitation to the production of mature pods but there were also significant losses at other developmental stages of fruit production. The results suggest that low seed set may be an adaptation of the species to an environment in which rainfall is scarce.     

A study of resources of Fagopyrum (Polygonaceae) native to China

Morphology, taxonomy, reproductive biology, esterase isozyme patterns and chromosome number were studied in 16 accessions of cultivated and wild buckwheat native to Tibet, Sichuan, Guizhou and Yunnan. Among these are four established species: Fagopyrum esculentum, F tataricum, F. pleioramosum and F gracilipes. The first three species are diploid (2 n = 16) and the last one is tetraploid (2 n = 32). In addition to the above, three new species are described: F. zuogongense, F. megaspartanium and E pilus. There are clear differences in esterase isozyme patterns between these species. The zymographs of esterase isozyme of diploid F. megaspartanium (native to Sichuan) and F. pilus (native to Tibet) are similar to those of F. esculentum and F tataricum respectively. F. gracilipes , in the small achene group of the genus, has a zymograph of esterase isozyme that is very different from the species in the large-achene group. The diploid accessions of F. pilus and F. megaspartanium may be ancestors of cultivated buckwheat.     

Ontogenetic differences in heterostylous plants and implications for development from a herkogamous ancestor

Abstract.— Alternative ontogenetic pathways among heterostylous species of Rubiaceae may reflect differences in their evolutionary histories. In this study, measurements were taken at different developmental stages on a series of long-styled (LS) and short-styled (SS) buds of the heterostylous taxa Psychotria chiapensis, P. poeppigiana , and Bouvardia ternifolia (all Rubiaceae). Results indicated that modifications in growth rates of stamens relative to corollas in all three species led to differences in anther heights between LS and SS flowers. Distinct style heights for LS and SS flowers of P. chiapensis and P. poeppigiana originate in the earlier stages of bud development and are maintained as styles elongate at equal rates. This contrasts with B. ternifolia , which has differences in style heights resulting from unequal relative growth rates between floral morphs. The "approach herkogamous" floral morphology, defined by having stigmas positioned above anthers, has been proposed as a potential evolutionary precursor for heterostylous taxa. To examine this hypothesis, floral development of two species with approach herkogamous morphologies, Psychotria pittieri and P. brachiata , was compared to that of the three heterostylous taxa. Differences in the relative rates of style elongation for flowers of approach herkogamous versus heterostylous species predict additional steps in the original model for the evolution of heterostyly from an approach herkogamous ancestor. The diversity of heterostylous ontogenies found within Rubiaceae provides insight into potential evolutionary pathways for this sexual system in other angiosperm families.     

Clonal growth of Lithospermum caroliniense (Boraginaceae) in contrasting sand dune habitats

The occurrence of clonal growth of distylous Lithospermum caroliniense was investigated in a population in the Nebraska Sandhills, an area where sand dunes have been relatively stable for at least 1500-3000 yr, and compared to a population occurring at the Indiana Dunes, an area of active sand dune formation. Spatial autocorrelation analysis indicated the occurrence of significant clonal propagation of genetically based floral morphs at Arapaho Prairie, but not for the Indiana Dunes. Apparent clonal growth in the Sandhills population had no overall negative effect on pollen deposition or fecundity relative to the Indiana population, although in some large clones the proportion of compatible pollen grains on stigmas was lower. Clonal growth may have occurred in the Sandhills population because of the greater age and stability of the Nebraska Sandhills; infrequent establishment of seedlings permits detection of clonal growth using the spatial pattern of floral morphs. At the Indiana dunes, repeated cycles of dune formation provide conditions favoring establishment of seedlings, and sand dune succession results in disappearance of L. caroliniense before the development of clones.     

Heterostyly in Erythroxylum coca (Erythroxylaceae)

Erythroxylum coca is a distylous species with a strong self-incompatibility system linked with the floral dimorphism. The two sets of stamens in the flowers are usually unequal in length, but between individuals of both morphs there is considerable variation in the relative qengths of the two sets of stamens, which is unrelated to the incompatibility system. Pin flowers produce more pollen grains than thrum flowers, but thrum pollen is larger than pin pollen. Within each morph the two sets of stamens produce pollen grains of slightly different diameter. Erythroxylum novogranatewe is also distylous. Pin flowers of E. novogranatense var. novogranatense are partially self-compatible, while thrum flowers of E. novogranatense var. truxillense are self-incompatible. Reports of tristyly and of four different morphs in species of Erythroxylum are probably misinterpretations, resulting from limited sampling, of the continuous variation in the relative lengths of the two sets of stamens.     

Contribution of small insects to pollination of common buckwheat, a distylous crop

Crop pollination by animals is an essential ecosystem service. Among animal-pollinated crops, distylous plants strongly depend on animal pollination. In distylous pollination systems, pollinator species are usually limited, although flowers of some distylous plants are visited by diverse animals. We studied the pollination biology of common buckwheat ( Fagopyrum esculentum ), a distylous crop mainly pollinated by honeybees and visited by many insect species, to evaluate the effects of non-honeybee species on pollination services. We focused on insects smaller than honeybees to determine their contribution to pollination. We applied pollination treatments with bags of coarse mesh to exclude flower visits by honeybees and larger insects and compared the seed set of bagged plants with that of untreated plants for pin and thrum flower morphs. We found a great reduction of seed set only in bagged pin flowers. We also confirmed that small insects, including ants, bees, wasps and flies, carried pin-morph pollen. These small insects transfer pollen from the short anthers of pin flowers to the short styles of thrum flowers, leading to sufficient seed set in thrum flowers. Consequently, small, non-honeybee insects have the potential to maintain at least half of the yield of this honeybee-dependent distylous crop.     

Quantifying reciprocity in distylous and tristylous plant populations

Reciprocal herkogamy (heterostyly) is an example of extreme relevance of accuracy of the location of the sexual organs within some floral systems. It involves the reciprocal positioning of anthers and stigmas in flowers of different plants within the same population, and the accuracy of this positioning is important to promote out‐cross pollination, prevent self‐interference, or both. Hence, several indices have been proposed to quantify reciprocity, most of them for populations with two different morphs (distylous). Here, we propose an extension of our index of reciprocity for distylous populations to be applied also to populations with three morphs (tristylous), allowing effective comparisons of reciprocity between tristylous, but also distylous populations. As for the distylous version, the index is based on comparison of the position of every single sexual organ in the sample with each and every organ of the opposite sex, for each of the three possible organ levels. Due to the massive amount of calculations required, a macro was developed that is available as Supplementary Information and at the website of the authors. The index and macro were tested on several hypothetical tristylous and distylous populations with predetermined mean and dispersion of sexual organs at each level, as well as on several actual tristylous and distylous populations. The index proposed is a solid tool for the study of reciprocity in distylous and tristylous populations. Comparisons between distylous and tristylous populations are easily performed and can be readily interpreted. The applicability of the index is facilitated through the software provided.     

异型花柱植物喀什补血草的传粉者功能群与花粉转移效率

异型花柱是受遗传控制的花柱多态现象, 被达尔文认为是植物通过在传粉者体表不同部位滞落花粉以促进型间花粉准确传递的一种适应。该现象虽已受到广泛关注, 但在一些花型变异较大且不稳定的传粉系统中, 不同传粉者对各花型繁殖所产生的影响仍知之甚少。该研究以分布于新疆天山南坡的一个有同长花柱共存的异型花柱植物喀什补血草(Limonium kaschgaricum)种群为研究对象, 对其花型构成及频率、传粉者及花粉转移效率等进行了调查分析。结果表明: 1)种群中除了存在雌/雄蕊长度交互对应的长(L)/短(S)花柱型花外, 还有雌/雄蕊同长的花(H型), 且各花型花的花冠口直径、花冠筒长及花粉量等参数间无差异, 但花粉纹饰和柱头乳突细胞形态具二型性。其中, H型花的花粉和柱头形态与L型花(或S型花)的一致。2)花型内和自花授粉均不亲和; 型间授粉时, 花粉和柱头形态不同的花型间亲和, 反之不亲和。3)种群内存在长/短吻两类传粉昆虫。在以短吻传粉者为主的盛花初、中期, L和H型花柱头上的异型花粉数均显著高于S型花的, 且L和S型花高位性器官间的异型花粉传递效率高于低位性器官间的; 而在以长吻传粉者为主的盛花后期, L和S型花的柱头间异型花粉数无显著差异, 且高/低位性器官间具有相同的异型花粉转移效率; 与传粉者出现时期相对应的、在花期不同阶段开放花的结实率也明显不同。4)长/短吻昆虫具明显不同的传粉功能, 短吻昆虫只能对L和H型花进行有效传粉, 且访花频率和型间花粉转移效率较低, 为低效传粉者; 而长吻昆虫对各花型均能有效传粉, 具高的访花频率和型间花粉转移效率, 为高效传粉者。因为长吻昆虫的阶段性出现所形成的不稳定传粉系统, 使低效的短吻昆虫可能会成为种群中花型变异的驱动力, 并使S型花受到更大的选择压力。H型花克服了柱头缩入的弊端, 可能会成为不稳定传粉系统下的一个替代花型而持续存在。