Robert A. Klips

The Ecology of Geitonogamous Pollination

Working as a natural historian in the 1700s, C.K. Sprengel wrote a pioneering book demonstrating that many hermaphroditic species require pollinator visits in order to produce seed (see Chapters 1 and 2). He did not provide a scientific explanation as to why cross-pollination is important, but in the next century Darwin, H. and F. Muller, and others proposed that various outcrossing mechanisms have evolved to avoid selfing and the consequences of inbreeding (Darwin, 1876; see Baker, 1983). Darwin also recognized that the potential for selfing is greatest in species with massive floral displays because having many flowers promotes the transfer of self-pollen to other flowers on the same genetic individual (geitonogamy). Following Darwin’s lead, many authors have suggested that the avoidance of selfing has been a major factor in the evolution of traits such as dioecy, self-incompatibility, monoecy, temporal separation of male and female organs (dichogamy), spatial separation of anthers and stigmas within flowers (herkogamy), and having few open flowers per day (see reviews by Arroyo, 1976; Lloyd, 1979; Bawa and Beach, 1981; Willson, 1983; Wyatt, 1983; Richards, 1986; Charlesworth and Charlesworth, 1987; Thomson and Brunet, 1990; de Jong, et al., 1992a; Harder and Barrett 1995; Hodges, 1995; also see Chapters 6, 8, and 14).

Delayed autonomous self-pollination in Hibiscus laevis (Malvaceae)

Delayed autonomous self-pollination allows outcrossing to occur while also ensuring that seeds are produced in the absence of pollen vectors. We investigated variation in the efficacy of this pollination mechanism in populations of Hibiscus laevis. Recurvature of stylar branches occurred after 1 d of anthesis, and in plants from Ohio, Illinois, Arkansas, and Oklahoma this behavior resulted in autonomous selfing (surprisingly, stylar movement was facultative in that it did not take place when the stigmas were already pollinated). In contrast to these more northern populations, the distance between anthers and stigmas was too great to allow autonomous selfing in plants from Texas, Mississippi, and Louisiana. Greenhouse studies of plants from Ohio demonstrated that autonomous selfing resulted in an average of 29.5 seeds per flower, as compared to 59.9 seeds per flower from hand-pollination of stigmas with self pollen. In an assessment of the possible significance of this selfing mode, emasculated flowers did not set significantly fewer seeds in a natural stand in Ohio, suggesting that few seeds resulted from autonomous selfing at that site. Modest inbreeding depression was detected at this population. Our results suggest that delayed autonomous selfing is more common in northern populations, where it may facilitate population establishment and persistence at times when pollinators are scarce.

Pollen competition as a reproductive isolating mechanism between two sympatric Hibiscus species (Malvaceae)

Differences in pollen tube growth rates (certation) between heterospecific (foreign) and conspecific pollen may strongly influence whether hybrid offspring are produced after mixed pollen loads are delivered to a stigma. For both members of a sympatric species pair, Hibiscus moscheutos and H. laevis, pollination by pure loads of foreign pollen resulted in fruit set that was not significantly different from conspecific pollination, indicating that pure loads of foreign pollen could readily result in hybrid offspring. However, the number of seeds per fruit from pure foreign pollinations was significantly less than that of pure conspecific pollination. Simultaneous mixed pollination resulted in a proportion of hybrid seeds (detected by an electrophoretic marker enzyme) that was significantly lower than expected based upon the capacity of foreign pollen to effect fertilization when applied in pure pollinations. After these 50/50% pollen mixtures were applied to stigmas, 8.0 and 7.4% hybrids were produced when H. moscheutos and H. laevis were the ovule parents, respectively. For these Hibiscus species, pollen competition appears to function as a barrier to hybridization that is of moderate intensity compared with similar barriers occurring between other recently studied sympatric species pairs.

Temporal and Geographic Variation in Predispersal Seed Predation on Hibiscus Moscheutos L. (Malvaceae) in Ohio and Maryland, USA

Abstract Seed predation has the potential to strongly reduce seed production and thereby act as a selective force on the evolution of flowering traits and other defenses against herbivory. We characterized levels of predispersal seed predation on Hibiscus moscheutos (Malvaceae) during 2001 and 2002 at four sites in Ohio and Maryland, USA. The seed predators were a weevil, Conotrachelus fissinguis (Coleoptera, Curculionidae) and a bruchid beetle, Althaeus hibisci (Coleoptera, Bruchidae). The weevil occurred at three of the four sites and damaged 24% to 94% of fruits in these populations. The bruchid occurred at all four sites, where it destroyed 4% to 27% of the seeds. Seed predation varied between years and among sites for both predators and year-by-site interactions were common. Variation in predation levels indicates that seed predators did not influence this species uniformly, but they were often abundant and sometimes destroyed nearly all of the seeds produced. At one of the Ohio sites, we assessed levels of seed predation at 5-d intervals during the 2001 flowering season. At this population, bruchid damage was greatest for seeds produced by flowers that opened in late July, when flowers were scarce, whereas weevil damage was greatest in mid-August and coincided with peak flowering. The timing and greater extent of weevil damage suggest that they may have a greater effect on plant fitness than bruchids.

DNA microsatellite analysis of sporophytes of the short-lived moss Physcomitrium pyriforme reveals a predominantly self-fertilizing mating pattern

Abstract Physcomitrium pyriforme (Funariaceae) is a monoicous moss with the potential for producing sporophytes either via outcrossing or intra-gametophytic self-fertilization. A core set of microsatellite markers was identified for use in population genetic studies of this species, and employed to ascertain its mating patterns. An initial collection of 88 sporophytes gathered from widely separated locations in a meadow in central Ohio, U.S.A. was screened for genetic uniformity based on trnL-F DNA sequences, and found to display considerable heterogeneity. Fifty-three members of the largest clearly defined clade were selected for genotyping at 6 variable microsatellite loci having expected heterozygosities ranging from 0.11 to 0.70. Fifty-two individuals (98.2%) were homozygous at all 6 loci, strongly indicative of self-fertilization. Only one individual (1.8%) appeared to be an outcrossed sporophyte, being heterozygous at 4 loci. Twenty-three samples that constituted a second well-marked clade displayed complex microsatellite genotypes strongly suggestive of a polyploid cytotype that, although not readily amenable to further analysis, is also not inconsistent with the predominantly selfing mating pattern exhibited by the 53 others. Male and female branches within gametophyte stems were observed to develop simultaneously or nearly so, and in 4 dense moss clusters within which samples of 8 gametophytes were genotyped, all 8 were found to be identical within each cluster. These results indicate that the overall mating pattern is predominantly selfing, and suggest that the species reproduces principally in what is, in effect, a clonal fashion, via the union of genetically identical gametes. This may be occurring both within and between gametophytes.

Aggregation, Oviposition and Emergence of Hibiscus Seed Beetles (Althaeus hibisci) at a Central Ohio Marsh

Abstract Several aspects of the reproductive ecology of the hibiscus seed beetle were investigated during summer 2003 and the succeeding winter and spring. Throughout the flowering period that extended from late Jul. to early Sept., aggregations of adult hibiscus seed beetles (Althaeus hibisci Olivier) were collected from swamp rose mallow (Hibiscus moscheutos L.) flowers at 5 d intervals and counted separately by sex. The occurrence of strongly male-biased sex ratios that decreased progressively during the flowering season, accompanied mid-season by slightly male-biased ratios on incipient fruits, suggests that both the flowers and developing fruits serve as sites frequented by the males to acquire mates. During the apparent peak of beetle activity, a cohort of incipient fruits was examined at 3 d intervals for the presence of beetle eggs. Oviposition began <3 d after corolla abscission and had occurred on nearly 100% of the fruits by day 6. Egg density reached an asymptote at day 12. Adult beetle emergence was monitored from seeds originating from flowers that bloomed during separate dates 5 d apart. The seeds were maintained until spring under weather conditions similar to those at the marsh where they were obtained. From seeds produced by fruits derived from flowers that bloomed early in the season (late Jul.–mid Aug.), beetles emerged in autumn of the same year. However, late-season fruits were colonized by beetles that emerged after mid-winter and thus effectively overwintered in the seed. This corrects previous reports indicating that beetles overwinter exclusively as emerged adults.