Diane L. Marshall

EFFECTIVE MATE CHOICE IN WILD RADISH: EVIDENCE FOR SELECTIVE SEED ABORTION AND ITS MECHANISM

Maternal plants that can affect the paternity of their seeds may influence the quality of their offspring. However, mate choice in plants has proved difficult to demonstrate because it is usually confounded by the effects of pollen competition and because its mechanisms are not well understood. We show here that stressed wild radish plants selectively abort seeds sired by particular donors. This selection occurs by position-dependent seed abortion and position-dependent resource allocation within fruits, coupled with position-dependent fertilization of ovules by pollen donors. The result is that stressed plants select for pollen donors that sire seeds in the basal and middle, rather than the stylar, thirds of fruits. Thus, in wild radish, the position of seeds fathered by pollen donors may act as a selective arena. Because of this position-dependent abortion, successful fathers sire the seeds most likely to mature and to be larger than average. However, siring these high-quality seeds may reduce the quantity of seeds sired by a donor after mixed pollination because pollen tubes must grow past fertilizable stylar ovules to reach middle and basal ovules, allowing pollen tubes from other donors to sire many seeds in the less favored positions. Since multiple paternity is common in wild radish fruits in the field, selective seed abortion probably occurs there as well. Finally, the position-dependent seed abortion necessary to this effective maternal choice occurs in other plants; thus, the mechanism suggested here may be a general mechanism of effective mate choice in plants that selects for position of fertilization, not just speed of pollen-tube growth.

PLASTICITY OF YIELD COMPONENTS IN RESPONSE TO STRESS IN SESBANIA MACROCARPA AND SESBANIA VESICARIA (LEGUMINOSAE)

The effects of limited moisture, limited nutrients, and defoliation on the subsequent allocation to yield components were measured in Sesbania macrocarpa and S. vesicaria in order to test the hypotheses that all yield components might be involved in the regulation of reproduction and that patterns of plasticity in reproduction may be species-specific. In S. vesicaria, the number of fruits per flower was the only yield component consistently reduced below the control. In S. macrocarpa, the responses to mild stress were reductions in the number of seeds per ovule and in seed weight, components of yield probably determined later than the number of fruits per flower. Severe stresses resulted in a change in flower number. Both the pattern and the amount of plasticity in yield components differed significantly between the species. Differences in responses to the same stresses may represent different adaptations resulting from variation in the importance of each component of yield for the successful reproduction of the two species in slightly different habitats. Within species, half-sib families differed in the amount, but not in the kind, of plastic response. Each species appears to be genetically invariant for plastic response in the same yield component, fruit survivorship, even though the overall pattern of plastic response differed between the species. Perhaps developmentally early yield components are genetically fixed to be either variable or stable, and responses in developmentally later yield components are determined by the levels of change in the earlier yield components.

ESTABLISHMENT PATTERNS OF NATIVE POPULUS AND SALIX IN THE PRESENCE OF INVASIVE NONNATIVE TAMARIX

Despite growing concern and research interest in plant invasions, we know very little about ecological processes involved during invasion episodes. Understanding these dynamics is especially important for habitats subject to reestablishment of historical disturbance regimes; because many invasive species respond positively to disturbance, interactions between colonizing natives and nonnatives at the seedling stage may determine success of invasions. In the American Southwest, reduced flood disturbance in this century has been associated with the invasion of nonnative Tamarix ramosissima, with Tamarix appearing to displace native flood-dependent species of Populus and Salix. We studied two riparian communities of these woody species that had established after over-bank flooding was allowed to occur in two protected areas in New Mexico. Growth and mortality were monitored for each individual within 1-m2 plots of seedling communities within these sites for 2–4 yr. Abiotic conditions measured for each plot in...

Seedling competition between native Populus deltoides (Salicaceae) and exotic Tamarix ramosissima (Tamaricaceae) across water regimes and substrate types

Populus deltoides subsp. wislizinii (Salicaceae), a cottonwood native to the Middle Rio Grande of New Mexico, must potentially compete against exotic Tamarix ramosissima (Tamaricaceae) during establishment after flooding. We investigated competitive interactions between seedlings of Tamarix and Populus in two substrates representing field textures and declining (i.e., draw-down) or stagnant water tables. The experiment was performed using a full-additive series design and interpreted with response surface models for each species. As reflected in both aboveground mass and height, Populus suppressed aboveground growth of Tamarix across all treatments, whereas competitive effects of Tamarix against Populus could only be seen at low Populus densities. Clay substrates with draw-down stimulated the greatest growth and created the most intense competitive environment for both species. Tamarix was competitively suppressed in every substrate tested, with the weakest response in sand with no draw-down, where growth of Populus was poorest. These results suggest that stream flow management that promotes Populus establishment could also aid in controlling Tamarix invasion across a range of substrates.

Do surface plant and soil seed bank populations differ genetically? A multipopulation study of the desert mustard Lesquerella fendleri (Brassicaceae).

Seed banks are an important component of many plant populations, but few empirical studies have investigated the genetic relationship between soil seeds and surface plants. We compared the genetic structure of soil seeds and surface plants of the desert mustard Lesquerella fendleri within and among five ecologically diverse populations at the Sevilleta National Wildlife Refuge in Central New Mexico. At each site, 40 Lesquerella surface plants and 40 samples of soil seeds were mapped and genetically analyzed using starch gel electrophoresis. Overall allele frequencies of soil seeds and surface plants showed significant differences across the five populations and within three of the five individual populations. Surface plants had significantly greater amounts of single and multilocus heterozygosity, and mean surface plant heterozygosity was also greater at the total population level and in four of the five individual populations. Overall soil seed (bot not surface plant) homozygosity was significantly greater than predicted by Hardy-Weinberg expectations at the total and individual population levels. Although F-alpha estimates revealed similarly small but significant genetic divergence within each life-history stage, estimates of coancestry showed that fine-scale (0.5-2 m) genetic correlations among the surface plant genotypes were roughly twice those of soil seed genotypes. An unweighted pair group method with arithrnetic mean cluster analysis indicated that in the two geographically closest sites, the surface plants were slightly more genetically similar to each other than to their own respective seed banks. We also found weak and/or negative demographic associations between Lesquerella soil seed and surface plant densities within each of the five sites. We discuss the difficulties involved with sampling and genetically comparing these two life-history stages.

PATTERNS OF MULTIPLE PATERNITY IN POPULATIONS OF RAPHANUS SATIVUS

Although multiple paternity has been documented for a large number of species, detailed studies of variation in multiple paternity among broods, individuals, and populations are lacking. We measured the extent of multiple paternity in the multi‐seeded fruits of wild radish, Raphanus sativus, from three natural populations in southern California. Every parent sampled produced one or more multiply sired fruits, and, in most plants, over half the fruits analyzed proved to be multiply sired. In all, 75% of the 388 multi‐seeded fruits analyzed showed multiple paternity. Among these fruits, the minimum number of paternal donors ranged from one to four, with a mode of two paternal parents. The fraction of multiply sired fruits varied from 68–85% among populations and from 40–100% among plants. Plants were heterogeneous for multiple paternity in the 1984 population. A significant positive correlation between multiple paternity and number of fruits per plant suggests that plants preferentially abort single sired fruits. The total number of mates that could be detected for entire plants ranged from 3–14 with a mode of seven. Multiple paternity is likely to be important in other species producing multi‐seeded fruits.

Postpollination effects on seed paternity: mechanisms in addition to microgametophyte competition operate in wild radish

After pollen arrives on a stigma, the paternity of seeds may be influenced by microgametophyte competition, maternal choice, genetic complementation between parents, and embryo competition. While microgametophyte competition has been well accepted, the other mechanisms are more difficult to demonstrate, and their effects are often confounded. Here, wild radish plants were pollinated with single and mixed pollen loads, and some plants were stressed such that reproduction was reduced. Effects of pollen donors, maternal families, maternal × paternal interaction, pollen donor number, and stress on fruit abortion, seed number per fruit, seed weight, and total seed weight per fruit were measured. Maternal‐plant × pollen‐donor interaction effects were found for all variables, indicating that genetic complementation or maternal choice occurred. Values of the components of reproduction were generally higher for multiply sired fruits than for singly sired fruits, indicating that either competition among embryos changed under multiple paternity or maternal choice for multiply sired fruits occurred. Finally, when maternal plants were stressed, the components of reproduction were more strongly affected by seed and fruit paternity. This result indicates that either competitive regimes among embryos were affected by stress or maternal plants become more selective under stress. In both cases where embryo competition might have been an explanation of the results, variation in seed weight within fruits was unaffected, suggesting that competitive regimes were unchanged. Clearly, mechanisms in addition to microgametophyte competition are important in sorting the pollen that arrives on stigmas of wild radish. These data suggest that maternal choice is likely to be important. In addition, these processes are likely to occur in the field, since the effects are stronger in stressed than in control plants.

PLASTICITY IN YIELD COMPONENTS IN RESPONSE TO FRUIT PREDATION AND DATE OF FRUIT INITIATION IN THREE SPECIES OF SESBANIA (LEGUMINOSAE)

SUMMARY (1) Changes in the components of yield in response to artificial predation and to date of fruit initiation were measured in three species of Sesbania. (2) The removal of fruits, which mimicked damage by chewing insects, resulted in significant changes in seed weight but not in seeds produced per fruit. Injection of alcohol into developing seeds, which mimicked damage by hemipterans, resulted in selective abortion of more heavily damaged fruits and of younger fruits. (3) The nature of the response to predation depended on its timing because responses become less flexible as development proceeds. (4) Differential responses to predation among the species appeared to be related to the investment in seeds versus pods, since S. vesicaria, which had the highest quotient of seed weight to fruit weight, had the highest rate of fruit abortion. (5) Fruits initiated on later dates were less likely to mature in all three species. Late fruits had an unchanged number of ovules but reduced seed size in S. macrocarpa (annual), reduced ovule number but unchanged seed size in S. vesicaria (annual), and unchanged ovule number and seed size in S. drummondii (perennial). (6) The differences among species in response to date of fruit initiation suggest that each annual species holds most constant the yield component most closely related to fitness, while the perennial is less responsive to stress because its stored reserves provide a buffer.

Mechanisms of differential pollen donor performance in wild radish, Raphanus sativus (Brassicaceae)

In order to understand the characters on which sexual selection might operate in plants, it is critical to assess the mechanisms by which pollen competition and mate choice occur. To address this issue we measured a number of postpollination characters, ranging from pollen germination and pollen tube growth to final seed paternity, in wild radish. Crosses were performed using four pollen donors on a total of 16 maternal plants (four each from four families). Maternal plants were grown under two watering treatments to evaluate the effects of maternal tissue on the process of mating. The four pollen donors differed significantly in number of seeds sired and differed overall in the mating characters measured. However, it was difficult to associate particular mechanistic characters with ability to sire seeds, perhaps because of interactions among pollen donors within styles or among pollen donors and maternal plants. The process of pollen tube growth and fertilization differed substantially among maternal watering treatments, with many early events occurring more quickly in stressed plants. Seed paternity, however, was somewhat more even among pollen donors used on stressed maternal plants, suggesting that when maternal tissue is more competent, mating is slowed and is more selective.

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

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.