Theresa M. Culley

The evolution of wind pollination in angiosperms.

Wind pollination (anemophily) of angiosperms probably evolved from insect pollination (entomophily) in response to pollinator limitation and changes in the abiotic environment. Recent evidence suggests that ambophily (a combination of both wind and insect pollination) might be more common than was previously presumed and could represent either a stable or transitional state. We review factors favoring the evolution of wind pollination and analyse the extent of specialization towards such pollination. Phylogenetic analyses suggest that anemophily is more likely to evolve in groups with small, simple flowers and dry pollen. Wind-pollinated plants appear to have a bimodal distribution in selfing rates, but further studies recognizing the possibility of a combination of wind and animal pollination within taxa are needed to clarify this relationship.

Population genetic structure of the cleistogamous plant species Viola pubescens Aiton (Violaceae), as indicated by allozyme and ISSR molecular markers

Few studies of genetic variation have focused on species that reproduce through both showy, chasmogamous (CH) flowers and self-pollinated, cleistogamous (CL) flowers. Using two different techniques, genetic variation was measured in six populations of Viola pubescens Aiton, a yellow-flowered violet found in the temperate forests of eastern North America. Results from eight allozyme loci showed that there was considerable genetic variation in the species, and population structuring was indicated by the presence of unique alleles and a θ (FST) value of 0.29. High genetic variation was also found using ISSR (inter-simple sequence repeat) markers, and population structuring was again evident with unique bands. Viola pubescens appears to have a true mixed-mating system in which selfing through CL and CH flowers contributes to population differentiation, and outcrossing through CH flowers increases genetic variation and gene flow among populations. Overall, allozyme and ISSR techniques yielded similar results, indicating that ISSR markers show potential for use in population genetic studies.

The Cleistogamous Breeding System: A Review of Its Frequency, Evolution, and Ecology in Angiosperms

Cleistogamy, a breeding system in which permanently closed, self-pollinated flowers are produced, has received increasing attention in recent years, but the last comprehensive review of this system was over 20 years ago. The goal of this paper is to clarify the different types of cleistogamy, quantify the number of families, genera, and species in which cleistogamy occurs, and estimate the number of times and potential reasons why cleistogamy has evolved within angiosperms. Cleistogamous species were identified through a literature survey using 13 online databases with references dating back to 1914; only those species well-supported by floral descriptions or empirical data were included in the data set. On the basis of this survey, we suggest the use of three different categories of cleistogamy in future studies: dimorphic, complete, and induced. Based on these categories, cleistogamy in general is present in 693 angiosperm species, distributed over 228 genera and 50 families. When analyzed on a family level across the angiosperms, the breeding system has evolved approximately 34 to 41 times. Theoretical investigations indicate that the evolution of cleistogamy in taxa may be influenced by the presence of heterogeneous environments, inbreeding depression and geitonogamy, and differential seed dispersal, as well as by various ecological factors and plant size. Cleistogamy will undoubtedly be discovered in additional species as the reproductive biology of more taxa is examined in the future. Such information will be invaluable for understanding the selective pressures and factors favoring the evolution of cleistogamy as well as the evolutionary loss of this breeding system, a subject that has received little attention to date.

A comparison of two methods of calculating GST, a genetic measure of population differentiation

G(ST) is a genetic statistic describing differentiation of populations and has frequently been compared with Hamrick and Godts (1989) review of the plant literature. We show here that some comparisons may be inappropriate if G(ST) was calculated in a different way than that used by Hamrick and Godt (HG). An alternative method advocated by Nei is mathematically different from the HG technique, occasionally resulting in different G(ST) values. We reviewed 695 studies that appeared between 1990 and September 1999 that cited Hamrick and Godt (1989) and found that many of these calculated G(ST) according to Neis method (46%), with the majority of these papers (61%) including comparisons to Hamrick and Godts review. We suggest that if G(ST) estimates are compared across studies, it is most appropriate to calculate them the same way. However, we found that in most cases, the magnitude of difference in G(ST) values was small, suggesting that qualitative comparisons of G(ST) estimates between most studies are probably valid. Nevertheless, we have identified theoretical and empirical situations in which large differences in G(ST) values are likely to arise. Thus, we advise future investigators to carefully consider which method to use in calculating G(ST) for a given data set.

The role of intraspecific hybridization in the evolution of invasiveness: a case study of the ornamental pear tree Pyrus calleryana

Hybridization between genetically distinct populations of a single species can serve as an important stimulus for the evolution of invasiveness. Such intraspecific hybridization was examined in Pyrus calleryana, a Chinese tree species commonly planted as an ornamental in residential and commercial areas throughout the United States. This self-incompatible species is now escaping cultivation and appearing in disturbed habitats, where it has the potential to form dense thickets. Using genetic techniques incorporating nine microsatellite markers, we show that abundant fruit set on cultivated trees as well as the subsequent appearance of wild individuals result from crossing between genetically distinct horticultural cultivars of the same species that originated from different areas of China. We conclude that intraspecific hybridization can be a potent but little recognized process impacting the evolution of invasiveness in certain species.

Long-term persistence of crop alleles in weedy populations of wild radish (Raphanus raphanistrum)

*Hybridization allows transgenes and other crop alleles to spread to wild/weedy populations of related taxa. Researchers have debated whether such alleles will persist because low hybrid fitness and linkage to domestication traits could severely impede introgression. *To examine variation in the fates of three unlinked crop alleles, we monitored four experimental, self-seeding, hybrid populations of Raphanus raphanistrum x Raphanus sativus (radish) in Michigan, USA, over a decade. We also compared the fecundity of advanced-generation hybrid plants with wild plants in a common garden experiment. *Initially, F(1) hybrids had reduced fitness, but the populations quickly evolved wild-type pollen fertility. In Year 10, the fecundity of plants from the experimental populations was similar to that of wild genotypes. Crop-specific alleles at the three loci persisted for 10 yr in all populations, and their frequencies varied among loci, populations and years. *This research provides a unique case study of substantial variation in the rates and patterns of crop allele introgression after a single hybridization event. Our findings demonstrate that certain crop alleles can introgress easily while others remain rare, supporting the assumption that neutral or beneficial transgenes that are not linked to maladaptive traits can persist in the wild.

ADAPTIVE RADIATION AND EVOLUTION OF BREEDING SYSTEMS IN SCHIEDEA (CARYOPHYLLACEAE), AN ENDEMIC HAWAIIAN GENUS1

Abstract A multi-disciplinary approach, including phylogenetic analysis, population biology, and quantitative genetics, has helped to elucidate the selective factors that have promoted speciation and shifts in breeding systems in Schiedea (Caryophyllaceae). Schiedea is the fifth largest lineage in the native Hawaiian flora and the most diverse lineage with respect to breeding systems. The genus is monophyletic and shares a common ancestor with a clade consisting of two arctic or boreal-north temperate species. Most inter-island colonizations were from older to younger islands, and most movement between islands led to sufficient isolation to result in formation of new species that are single-island endemics rather than species with multi-island distributions. Closely related species pairs occurring on older islands tend to differ in habitat and are isolated ecologically on the same island, while species pairs on younger islands tend to be in similar habitat on different islands. Speciation within this lineage has been associated with shifts in habitat, pollination system, and breeding system, including evolution of selfing (obligate autogamy, facultative autogamy), mixed mating systems, and dimorphism (gynodioecy, subdioecy, and dioecy). Dimorphic breeding systems appear to have been derived independently twice in Schiedea, and facultative autogamy and obligate autogamy have both evolved three times. The colonization of windy, dry habitats appears to occur before changes in sex allocation patterns, and the evolution of dimorphism in this lineage has been promoted by the combination of high inbreeding depression and high selfing rates. Many morphological traits associated with allocation to male and female function are highly heritable, and genetic correlations in general do not appear to constrain the evolution of dimorphism in Schiedea.

The beginning of a new invasive plant: a history of the ornamental Callery pear in the United States.

ABSTRACT The Callery pear (Pyrus calleryana Dcne. [Rosales: Rosaceae]), an ornamental tree from China, has begun appearing in disturbed areas throughout the United States. To understand the relatively recent spread of this species into natural areas, we review its horticultural history, the traits promoting its invasiveness, and its current invasive status. Cultivated varieties (cultivars) of this species sold in the United States originate from different areas in China and represent genotypes that have been planted in high densities in residential and commercial areas in the introduced range. The species cannot self-pollinate because of a self-incompatibility system, but recent fruit set is due to crossing between different cultivars or between the scion and rootstock of cultivated individuals. Consequently, individual cultivars themselves are not invasive, but the combination of cultivars within an area creates a situation in which invasive plants can be produced. Because of the established nature of this species in urban areas, the spread of wild P. calleryana will most likely continue, especially as new cultivars continue to be introduced into the mixture of cultivars already present.

Inbreeding depression and selfing rates in a self-compatible, hermaphroditic species, Schiedea membranacea (Caryophyllaceae)

Inbreeding depression and selfing rates were investigated in Schiedea membranacea (Caryophyllaceae), a hermaphroditic species endemic to the Hawaiian Islands. Most theoretical models predict high inbreeding depression in outcrossing hermaphroditic species and low inbreeding depression in inbreeding species. Although high outcrossing rates and high levels of inbreeding depression are characteristic of many species of Schiedea, self- fertilization is common among relatives of hermaphroditic S. membranacea, and high selfing rates and low levels of inbreeding depression were predicted in this species. Sixteen individuals grown in the greenhouse were used to produce selfed and outcrossed progeny. Inbreeding depression, which was evident throughout the stages measured (percentage viable seeds per capsule, mean seed mass, percentage seed germination, percentage seedling survival, and biomass after 8 mo), averaged 0.70. Inbreeding depression among maternal families varied significantly for all measured traits and ranged from -0.12 to 0.97. Using isozyme analysis, the multilocus selfing rate varied from 0.13 to 0.38 over 4 yr. Contrary to the initial prediction of high selfing and low inbreeding depression based on phylogenetic relationships within Schiedea, low selfing rates and high levels of inbreeding depression were found in S. membranacea. These results indicate that outcrossing is stable in this species and maintained by high levels of inbreeding depression.

FECUNDITY SELECTION IN A SUNFLOWER CROP–WILD STUDY: CAN ECOLOGICAL DATA PREDICT CROP ALLELE CHANGES?

Genes that spread from transgenic crops to populations of weedy relatives can be a cause of concern if fitness-related, transgenic traits persist and enhance weed invasiveness. Studies of the prevalence of crop-specific genetic markers in wild populations can provide data on such introgression. We conducted a field experiment in eastern Kansas to measure changes in frequencies of crop-specific genetic markers in wild sunflower (Helianthus an- nuus). Three allozyme markers were monitored in three artificial populations that each initially consisted of 100 wild and 100 F1 hybrid plants. Survival, flowering time, and average seed production per plant were quantified during the first year of the study (1997). Hybrid plants produced only 1-2% as many seeds per plant as wild plants but did not differ in survivorship. Simple selection models incorporating fecundity differences between wild and F1 hybrids accurately predicted crop allele frequencies in the 1998 seedlings. We predicted that fre- quencies of crop alleles in 1998 would average ;0.03 for the three populations. Crop-specific allele frequencies dropped from the initial level of 0.25 to a mean of 0.03 in the 1998 seedlings and averaged 0.05 in the next generation of seeds. Accounting for differences in flowering phenology and predispersal seed predation did not improve the accuracy of our predictions for 1998 seedlings. Our results suggest that ecological data can be useful for estimating the frequencies of crop genes following episodes of crop-wild hybridization in sunflowers. This approach can be applied to other study systems in which data on survivorship and fecundity are used to estimate a genotypes evolutionary fitness.