Evolutionary consequences of extensive morph loss in tristylous decodon verticillatus (Lythraceae): a shift from tristyly to distyly?

The evolution of distyly from tristyly has occurred repeatedly, especially in the Lythraceae. However, the evolutionary forces involved remain unclear since species exhibiting transitional stages between tristyly and distyly have rarely been studied. The self-compatible, wetland perennial Decodon verticillatus (Lythraceae) may provide this transitional variation since populations commonly lack style morphs, particularly the mid-styled (M) morph. In dimorphic populations lacking the M morph, anthers positioned at the mid level in both the long- (L) and short-styled (S) morphs have lost their target stigma, setting the stage for either evolutionary repositioning of mid-level anthers to increase pollen export to L and S stigmas, or increased variability in mid-level anther position resulting from relaxed selection. We examined these two hypotheses by comparing floral morphology in eight dimorphic and ten trimorphic populations from throughout the species’ range. We found no evidence that loss of the M morph has led to evolutionary modification of mid-level stamens. While mid-level stamens of the S morph were 11.0 ± 4.0% (mean ± 1 SE) longer than those of the L morph in dimorphic populations, divergence in stamen length between morphs occurred to the same extent (10.4 ± 2.0%) in trimorphic populations and cannot be attributed to the absence of the M morph. Analyses of variability using median ratio tests revealed no difference in the variability of mid-level stamen length between dimorphic and trimorphic populations. Mid-level stamens were not more variable than long- and short-level stamens within dimorphic populations. The consistent divergence in mid-level stamens between the L and S morphs may reflect morph-specific differences in the optimal position of mid-level anthers for maximizing cross-pollination and avoiding self-fertilization.     

STOCHASTIC LOSS OF STYLE MORPHS FROM POPULATIONS OF TRISTYLOUS LYTHRUM SALICARIA AND DECODON VERTICILLATUS (LYTHRACEAE)

Despite the theoretical importance of stochastic processes in evolution there have been few empirical studies of the interaction between genetic drift and selection on the maintenance of polymorphisms in plant populations. We used computer models to investigate the interaction between drift and frequency-dependent selection in affecting style morph frequencies in populations of tristylous species. Drift produces a distinct pattern of morph frequency variation involving: 1) the loss of the S morph and, to a lesser extent, the M morph; 2) no consistent bias in frequencies within populations; 3) a restricted pattern of variation involving a deficiency of one morph and equal excesses of the other two. Morph frequencies were surveyed in 137 populations of Lythrum salicaria from both its native range in Europe (N = 35) and recent adventive range in Ontario (N = 102), and 133 populations of Decodon verticillatus from four regions in eastern North America with different glacial histories to assess these theoretical predictions. There was a negative relationship between morph loss and population size in both species; the relationship was weaker in D. verticillatus than in L. salicaria. Morph loss was more frequent in the adventive than native range of L. salicaria, and in populations of D. verticillatus from glaciated northern regions compared with the unglaciated southern portion of its range. Simulations incorporating variation in life history, regeneration strategy and mating patterns revealed that the degree of morph loss was strongly influenced by year to year survival, clonal propagation, self-fertilization and departures from disassortative mating. Comparing the pattern of morph frequency variation between species supported these predictions. Morph loss was lower in self-incompatible L. salicaria (0% in Europe; 23% in Ontario), which reproduces through seed compared to self-compatible, clonal D. verticillatus (52%). A stochastic model provides the most parsimonious explanation for observed patterns of morph frequency variation in both species.     

荧光标记在植物花粉管构造及生长特性研究中的应用

花粉(管)构造及花粉管生长特性长期以来一直是人们研究的焦点,近年来,随着生物学和新材料不断发展,新荧光标记技术的运用,植物花粉管生长特性的研究取得巨大进步.本文介绍了花粉管生长特性等研究中用到的主要荧光标记物质,包括:苯胺蓝、Flou-3、罗丹明鬼笔环肽、TUNEL标记以及荧光蛋白等的理化性质、作用原理,并对应用荧光标记探索植物花粉管构造及生长特性的研究进展进行了综述.     

Pollen–pistil interaction in protogyny and self-incompatibility system of Indian mustard (Brassica juncea (L.) Coss.)

The pollen morphology of two species of Drosera has been investigated by means of both light and electron microscopy. The apertures are situated proximally, a state rarely found in the pollen of Angiospermous plants. The pollen morphology of Aldrovanda, Dionea and Drosophyllum is compared.     

Pollen donor performance can be consistent acrossmaternal plants in wild radish (Raphanus sativus,Brassicaceae): a necessary condition for the action of sexualselection

The possibility that sexual selection has played a role in theevolution of plant reproductive characters remains interesting, butcontroversial. One reason is that clear demonstration of the necessaryconditions for sexual selection is lacking. For sexual selection tooccur, differences in pollen donor performance must be consistent acrossmaternal plants and not be due to mechanisms such asself-incompatibility that produce interactions between maternal plantsand pollen donors. Here, I performed two experiments with wild radish totest whether differences in pollen donor performance are consistentacross maternal plants and not due to subtle effects of theincompatibility system. In the first, all maternal and paternal lineageshad different S-alleles. There were 16 maternal plants, four in each offour lineages and four pollen donors, one in each of four lineages. Rankorder of pollen donor performance, in terms of number of seeds siredafter mixed pollination, was highly consistent across maternal plantsand maternal lineages. In addition, maternal stress treatment had aneffect on mating success of pollen donors, but the effect was subtle anddid not affect the rank order of seeds sired by the four pollen donors.In the second experiment, pollinations were performed on both mature andimmature stigmas. Immature stigmas allowed some self seed set, so theincompatibility system was compromised. There was some nonrandom seedpaternity on both mature and immature stigmas. However, the amount ofnonrandom mating was less on immature stigmas. Taken together, theseexperiments show that the kind of consistent nonrandom mating necessaryfor sexual selection occurs in wild radish, but that the incompatibilitysystem and the mechanisms for sorting among compatible mates may overlapin time of development or in somepathways.     

DOES INTERFERENCE COMPETITION AMONG POLLEN GRAINS OCCUR IN WILD RADISH?

Interest in the possibility of sexual selection in plants has focused primarily on competition among pollen donors based on the speed of pollen-tube growth. However, when pollen arrives on stigmas, there is the opportunity for both races for access to ovules (exploitation competition) and interference with the germination and growth of pollen from other donors (interference competition). We considered whether this second form of competition might occur among pollen grains of wild radish in two experiments. In the first, interference likely occurred because the amount of pollen germination was less in mixed-donor than in single-donor pollinations. This result was duplicated in a second experiment, which also showed that interference occurred only when pollen grains from different donors were in direct contact with each other. In addition, in the second experiment, the opportunity for interference affected the frequency of seeds sired by different pollen donors. Because pollen loads are often mixed in nature, interference competition among pollen grains may be important in the ecology and evolution of plant reproduction.     

The absence of cryptic self-incompatibility in Clarkia unguiculata (Onagraceae)

Many species exhibit reduced siring success of self-relative to outcross-pollen donors. This can be attributed either to postfertilization abortion of selfed ovules or to cryptic self-incompatibility (CSI). CSI is a form of self-incompatibility whereby the advantage to outcross pollen is expressed only following pollinations where there is gametophytic competition between self and outcross pollen. Under the definition of CSI, this differential success is due to the superior prefertilization performance (pollen germination rate and pollen tube growth rate) of outcross pollen relative to self pollen. Although CSI has been demonstrated in several plant species, no studies have assessed among-population variation in the expression of CSI. We conducted a greenhouse study on Clarkia unguiculata (an annual species with a mixed-mating system) to detect CSI, and we compare our observations to previous reports of CSI in C. gracilis and another population of C. unguiculata. In contrast to these previous studies of CSI in Clarkia, we used genetic rather than phenotypic markers to measure the relative performance of selfed vs. outcross pollen. In this study, we measured the intensity of CSI in C. unguiculata from a large population in southern California and we determined whether the magnitude of pollen competition (manipulated by controlling the number of pollen grains deposited on a stigma) influenced the outcome of competition between self and outcross pollen. In contrast to previous investigations of Clarkia, we found no evidence for CSI. The mean number of seeds sired per fruit did not differ between self and outcross pollen following either single-donor or mixed pollinations. In addition, the relative success of selfed vs. outcross pollen was independent of the magnitude of pollen competition. These results suggest that: (1) one of the few nonheterostylous species previously thought to be cryptically self-incompatible is completely self-compatible (at least in the population studied here) or (2) phenotypic markers may be problematic for the detection of CSI.     

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.     

High paternal diversity in the self-incompatible herb Arabidopsis halleri despite clonal reproduction and spatially restricted pollen dispersal

The number of sires fertilizing a given dam is a key parameter of the mating system in species with spatially restricted offspring dispersal, since genetic relatedness among maternal sibs determines the intensity of sib competition. In flowering plants, the extent of multiple paternity is determined by factors such as floral biology, properties of the pollen vector, selfing rate, spatial organization of the population, and genetic compatibility between neighbours. To assess the extent of multiple paternity and identify ecological factors involved, we performed a detailed study of mating patterns in a small population of a self-incompatible clonal herb, Arabidopsis halleri . We mapped and genotyped 364 individuals and 256 of their offspring at 12 microsatellite loci and jointly analysed the level of multiple paternity, pollen and seed dispersal, and spatial genetic structure. We found very low levels of correlated paternity among sibs ( P _full-sib = 3.8%) indicating high multiple paternity. Our estimate of the outcrossing rate was 98.7%, suggesting functional self-incompatibility. The pollen dispersal distribution was significantly restricted (mean effective pollen dispersal distance: 4.42 m) but long-distance successful pollination occurred and immigrating pollen was at most 10% of all pollination events. Patterns of genetic structure indicated little extent of clonal reproduction, and a low but significant spatial genetic structure typical for a self-incompatible species. Overall, in spite of restricted pollen dispersal, the multiple paternity in this self-incompatible species was very high, a result that we interpret as a consequence of high plant density and high pollinator service in this population.