DISTANCE-DEPENDENT REGULATION OF STAMEN NUMBER IN CROSSES OF SCLERANTHUS ANNUUS (CARYOPHYLLACEAE) FROM A DISCONTINUOUS POPULATION

Scleranthus annuus is a highly inbreeding annual that has varying numbers of fertile stamens per flower. Two stamen-positions always have fully fertile stamens, whereas the other eight carry staminoids or stamens to varying degrees. I measured male expression in progeny produced by crossing individuals growing in a discontinuous population. Four types of progeny were analyzed: from self-pollinations, from cross-pollinations within a patch, from cross-pollinations between patches, and from cross-pollinations between populations. Selfed progeny showed the lowest total male fertility (25.8), followed by between-population crosses (26.7), between-patch crosses (27.4), and within-patch crosses (27.8). The effect of crossing, as measured by the relative increase in frequency of fully fertile stamens compared to selfed progeny, is highest for within-patch crosses and declines with increasing spatial separation between parents. The increase was strongest for one of the antipetalous stamen positions in progeny produced by between-patch crosses (490%). The response to crossing measured as an increase in stamen fertility was not the same for all ten stamen positions. A strong increase of fertile stamens is noted in all types of crossed progeny for the five stamen positions in the outer whorl (antipetalous stamens), positions that in selfed progeny carry staminoids. The three positions in the inner whorl that are not occupied by fully fertile stamens show varying responses to crossing.     

The effects of fire on soil nitrogen associated with patches of the actinorhizal shrub Ceanothus cordulatus

Nitrogen is a limiting resource in many temperate forests and nitrogen-fixing plants are usually limited to the early stages of post-disturbance succession. In fire-dependent Sierra Nevada forests, however, Ceanothus cordulatus is relatively abundant even in old-growth forest conditions which are at least partly maintained by fire. We conducted a field experiment to determine if soil beneath Ceanothus patches represent `resource islands' of available N which persist after fire. Nine plots containing discrete patches of Ceanothus, Arctostaphylos patula (manzanita; chosen as a non N-fixing reference species), and bare forest floor were subjected to either a low-intensity (n=3) or high-intensity (n=3) bum treatment, or remained unburned as controls (n=3). Soil temperatures during the bum were monitored by a network of thermocouples placed at the surface of the mineral soil and at ca. 10 cm depths. Soil samples were collected from the organic horizon, 0-10 cm and 15-25 cm depths within each patch type immediately before burning and 2 days, and 6, and 11 months after. Soil moisture, total C and N, and ammonium and nitrate concentrations were determined in the laboratory. Before the burn, Ceanothus patches were significantly enriched in total and inorganic N in the organic horizon relative to the other patch types. A sharp increase in inorganic N was observed in all patch types and depths immediately following burning, but by 6 months after the burn, Ceanothus patches were significantly enriched relative to the surrounding patch types and remained so at months. Resprouting Ceanothus patches will continue to be an important source of a limiting nutrient in this fire-prone ecosystem.     

Landscape context outweighs local habitat quality in its effects on herbivore dispersal and distribution

Past studies with spatially structured herbivore populations have emphasized the primacy of intrinsic factors (e.g., patch quality), patch geometry (e.g., patch size and isolation), and more recently landscape context (e.g., matrix composition) in affecting local population abundance and dispersal rate. However, few studies have examined the relative importance of each factor, or how they might interact to affect herbivore abundance or dispersal. Here, we performed a factorial field experiment to examine the independent and interactive effects of patch quality (plant biomass, leaf protein, leaf phenolics) and matrix composition [mudflat or non-host grass (Bromus inermis)] on planthopper (Prokelisia crocea) emigration from host-plant patches (prairie cordgrass, Spartina pectinata). In addition, a field survey was conducted to examine the relative importance of patch quality, geography, and matrix composition on planthopper occupancy and density. In the experiment, we found that rates of emigration from low and intermediate quality patches were, on average, 21% percent higher for patches embedded in brome than mudflat. In contrast, the emigration rate was unaffected by matrix composition in nutrient-rich patches. Within matrix types, plant quality had little effect on emigration. In the survey, planthopper density and the patch occupancy rate of planthoppers increased nonadditively with increasing patch size and the percentage of the surrounding matrix composed of mudflat. This study suggests that landscape-level factors, such as the matrix, may be more important than factors intrinsic to the patches.     

Genetic constitution of germ cells in intervarietal and interspecific chimeras of Brassica induced by in-vitro grafting

The characteristics of intervarietal and interspecific chimeras synthesized by the graft-culture method were determined by morphology, anthocyanin pigmentation pattern, and crossing. In an intervarietal chimera between YR-ranpou (green cabbage) and Ruby ball (red cabbage) in Brassica oleracea, a segregation phenomenon was noted in which seeds giving rise to purple and green plants were both produced in a single capsule in F₁ progeny from crosses of chimeras with YR ranpou, the anthocyanin-free graft partner type. The degrees of segregation varied, reflecting the structure of the chimeras. YR ranpou-dominant chimeras produced capsules in which seeds gave rise to green plants at a high frequency, while Ruby ball-dominant chimeras produced capsules in which seeds in one capsule gave rise to purple plants at a high frequency. Mixed chimeras produced capsules with green plants or purple plants more regularly than did other chimeral types. Furthermore, a chimeral type in which seeds gave rise to green and purple plants was found in 3.2% of the total crosses. Segregation patterns in the progenies corresponded with the chimeral types. Chlorophyll-deficient variation (resulting in variegation or the production of albino plants) was found at a frequency of 2.6%. These results show that chimeric tissues are actually in a mixed state and that either the ovary develops from more than two cells or else that variation occurs in the germ-cell layer. In interspecific chimeras between Ruby ball and Komatsuna (B. campestris) various types of chimeras generally showed low pollen fertility, few capsules, and low seed-setting. Progenies from selves (geitonogamy), open crosses and crosses with the two parental species produce a predominantly homogeneous genotype showing either the Ruby ball or the Komatsuna type. Only two crosses produced four interspecific hybrids which expressed variations in their morphological and isozymic characters.     

Ribosomal DNA inheritance and recombination in Neurospora crassa

Summary The genetic segregation of ribosomal DNA (rDNA) in Neurospora crassa was analyzed by exploiting restriction fragment length polymorphisms in the nontranscribed spacer (NTS) sequences of nine laboratory wild-type strains and wild-collected strains. In an analysis of random spore progeny from seven crosses, and of ordered tetrads from two of those crosses the rDNA was shown to be inherited in a simple, stable Mendelian fashion, exhibiting an approximately 1:1 ratio of the two parental rDNA types. No meiotic recombinants were detected among the progeny, indicating that non-sister-chromatid crossing over is highly suppressed in the rDNA region. The basis for this suppression of meiotic recombination is not known.     

Gibbon foraging decisions and the marginal value model

We use data from an observational field study of frugivory in two sympatric gibbons, lar (Hylobates lar) and siamang (H. syndactylus), to test assumptions and predictions of the marginal value model (MVM). A key prediction of the MVM is that marginal gain rates at the time of leaving the patch are equal across patch types. We found that this is not the case for gibbons: rates of energy intake at the end of feeding sessions were significantly different for different types of fruit, and we could not attribute this to temporal variation in fruit availability. Initial and final caloric intake rates were highly correlated. This suggests that gibbons do not adjust the time spent in patches in order to maximize the average rate of energy intake. Similar results were obtained for all other currencies considered. Gibbon foraging appears to satisfy several, but not all, assumptions of the MVM. As required by the model, fruit patches occur as discrete units, patches are encountered sequentially, travel time between patches exceeds search time between items within a patch, search for and search within patches are incompatible activities, and intake rates decline over time spent in a patch. However, the declining rates we detected may be an effect of satiation instead of patch depletion, patches probably are not encountered at random, and group members may not forage independently. Thus, our results suggest that the MVM is not an adequate model of gibbon foraging behavior, but they do not invalidate the MVM per se.     

Growth performance of hybrid families by crossing selfed lines of Betula pendula Roth

Hybrid breeding is an effective approach in many agricultural crops. In allogamous tree species severe inbreeding depression and long reproductive cycles generally prohibit its use. However, three generations of selfing in silver birch (Betula pendula Roth) were obtained by forcing trees to flowering under greenhouse conditions. Hybrids were produced by crossing first-, second and third-generation selfed lines. The effects of different levels of parental inbreeding on the growth performance of hybrid families were observed in a 9-year-old field progeny test. Also, provenance crosses were carried out between selfed lines from different parts of Finland and several other European countries. Observations of growth performance of the provenance hybrids were made in the same trial. The results indicated that the mean stem volumes were significantly different between classes of parental in breeding coefficients (F_P) (P<0.0001), and were positively correlated with F_P (r=0.9106, P<0.05). Within-family variation of the hybrid families decreased with an increase of F_P. The performance of the provenance crosses between parents at a relatively close distance did not depart significantly from the standard controls. However, when the cross distance was extended far to the south, hybrids grew faster, indicating either higher heterozygosity or an extended growth period.     

Pollen tube expression of pseudo-self-compatibility (PSC) inPetunia hybrida

Summary AnS _1.1 self-incompatible (SI) petunia plant which showed atypical seed set was found in an I₇ population. This plant showed a strong SI reaction when selfed but produced varying amounts of seed when used as the seed parent in crosses with unrelated individuals homozygous for the sameS allele. Reciprocal crosses yielded no seed indicating that the reaction was a stylar response. Self seed obtained by high temperature treatments produced 18 plants, all of which exhibited the parental characteristics, the ability to reject self pollen but accept, to varying degrees, pollen bearing the sameS allele from unrelated plants. Several petunias homozygous forS ₁, and exhibiting various levels of PSC as determined by self seed set, progeny tests and temperature treatments, were used as pollen parents. The mean seed set of these crosses produced a ranking of the pollen parents which reflected the PSC levels obtained by other methods. The behavior of the F₁ and F₂ populations suggests that the pollen discriminating ability may be a simply inherited, dominant character in these plants. The styles of these unusual petunias illustrate the participation of the pollen tube in determining PSC.Scientific Journal Series Paper Number 10.479 of the Minnesota Agricultural Experiment Station     

Does intraspecific hybridization contribute to the evolution of invasiveness?: an experimental test

One of the major objectives of research on invasive species is to determine the relative importance of different evolutionary and ecological forces in the invasion process. It was recently suggested that post-introduction intraspecific hybridization between previously isolated genotypes could produce novel and/or heterotic progeny that might express enhanced invasiveness. We tested this hypothesis with Silene latifolia, a European native that has successfully invaded North America and has previously been shown to have undergone genetic change since its introduction. In a common garden experiment we compared the performance of plants derived from within and between population crosses from eight European and 18 North American populations. Results showed that there was no significant effect of crossing distance on progeny phenotype. Furthermore, progeny from within or between population crosses did not differ in size, reproductive output or survival. Collectively, these results suggest that the invasive phenotype of S. latifolia is likely the result of natural selection and/or genetic drift rather than intraspecific hybridization.     

Hybridization studies in the genusSilene sectt.Siphonomorpha andAuriculatae (Caryophyllaceae)

Ten species in the genusSilene sectt.Siphonomorpha andAuriculatae were crossed artificially involving 612 crosses to test inter- and infraspecific, intervarietal and intersectional crossability. In sect.Siphonomorpha all interspecific crosses (between diploids) failed due to cross- or seed-incompatibility; however, intervarietal crosses betweenS. gigantea var.gigantea andS. gigantea var.incana produced hybrids. In sect.Auriculatae hybrids were produced betweenS. vallesia andS. boryi, both tetraploids, but crosses between these and the diploid species were unsuccessful. The delimitation and status of the species in both sections was supported by the crossing results.     

Patch-Leaving Decision of the Predatory Mite Amblyseius womersleyi (Acari: Phytoseiidae) Based on Multiple Signals from Both Inside and Outside a Prey Patch

We compared the emigration rates of Amblyseius womersleyi from prey patches (leaf disks) of different conditions in airflow containing either infested plant volatiles (volatiles airflow) or uninfested plant volatiles (control airflow). Both airflow and prey patch conditions significantly affected the emigration rates. Emigration rates from patches carrying prey products (feces, exuviae, webs, etc.) and prey eggs were significantly lower in control airflow than in volatiles airflow. Under other patch conditions, the rates were lower in control airflow than in volatiles airflow, although the difference was not significant. In both airflows, the lowest emigration rates were observed when predators were in a heavily infested patch. Patches carrying prey products and prey eggs resulted in lower emigration rates than patches carrying eggs alone and patches emitting prey-infested plant volatiles but carrying no prey. Thus, A. womersleyi appears to decide the timing of emigration based on two criteria: prey products in the patch and prey-infested plant volatiles from outside.     

Field and genetic studies testing optimal outcrossing in Agave schottii,a long-lived clonal plant

In thisstudy we combine field experiments, designed to test the predictions of optimal outcrossing theory in Agave schottii, with molecular genetic studies, using RAPD (random amplified polymorphic DNA), polymerase chain reaction to assess the underlying genetic hypothesis of optimal outcrossing theory. Initially, 48 females of A. schottii were hand-pollinated with pollen collected from 1 m, 10 m, 100 m, and 2500 m distances. Each female received all four distance treatments. Additionally, a subset of the focal females and their pollen donors were used in an analysis of genetic similarity across the four distances. Results of hand-pollinations showed that crosses of 1 m had significantly lower seed set than 10 m and 100 m crosses. Crosses of 2500 m had intermediate seed set. Combined relative fitness was significantly lower for 1 m crosses compared to 10 m crosses, while 100 m and 2500 m crosses were intermediate. Thus, A. schottii experiences inbreeding depression and a trend toward outbreeding depression. Genetic analyses showed a similar pattern: individuals 1 m apart had on average higher genetic similarity (proportion of bands shared) than individuals separated by greater distances, with a trend toward lower genetic similarity for plants located 2500 m distant. The observed spatial genetic patterns are likely maintained by the combined effects of clonal reproduction, clone longevity, limited seed dispersal and the substantial number of inbred progeny produced, counteracting distant allele transfer which tends to reduce population genetic structure. The correspondence between our ecological and genetic results indicates that RAPD markers are useful tools for assessing ecological phenomena.     

Interpatch movement of the red milkweed beetle,Tetraopes tetraophthalmus: individual responses to patch size and isolation

Individual movement patterns and the effects of host plant patch size and isolation on patch occupancy were examined for red milkweed beetles, Tetraopes tetraophthalmus, residing in a heterogeneous landscape. Male beetles were found to move both more often and farther between host plant patches than female beetles, and this difference affected the patterns of patch occupancy observed. Overall, unoccupied milkweed patches were smaller and more isolated than patches occupied by beetles. Patches uninhabited by females tended to be more isolated, but not necessarily smaller, than patches with female beetles, indicating that females may be affected more by patch isolation than patch size. Presence of male beetles on patches showed a stronger response to patch size than to patch isolation. Differences in movement between males and females illustrate the need for demographically based dispersal data. Comparisons of Tetraopes interpatch movement patterns between landscapes composed of patches of different size revealed that landscapes with overall smaller patches may have greater rates of interpatch movement.     

Pollen selection for low temperature adaptation in tomato

Summary Pollen selection experiments were conducted in tomato to determine the effects of low temperature conditions during pollination on the rate of root elongation of the progeny. Pollen was harvested from an F₁ interspecific hybrid between a high altitude Lycopersicon hirsutum accession and the cultivated tomato L. esculentum. The pollen was applied to stigmas of malesterile L. esculentum plants maintained in growth chambers set at either 12°C/7°C or 24°C/18°C. BC₁ seeds from the low and normal temperature crosses were germinated and root elongation rate was measured at either 9°C or 24°C. At 9°C, the rate of root elongation for progeny of the low temperature crosses was higher than for progeny of crosses at normal temperatures; at 24°C the rate of root elongation was similar for the two crossing treatments. To compare the temperature responses of the two backcross populations we also calculated the relative inhibitory effect of low temperature on the rate of root elongation: the ratio between the rate of root elongation at 9°C to that at 24°C. Root elongation of seedlings from the low temperature crosses was less inhibited by the cold than root elongation for progeny of the normal temperature crosses. These results suggest a relationship between pollen selection at low temperatures and the expression of a sporophytic trait under the same environmental stress.     

Reproductive systems and crossing potential in three species ofBulbophyllum (Orchidaceae) occurring in Brazilian ‘campo rupestre’ vegetation

Experimental self-, cross- and interspecific pollinations were carried out inBulbophyllum weddellii, B. involutum andB. ipanemense (Orchidaceae). The last two species are closely related, butB. weddellii is somewhat more distant. The three species are self-compatible, with pollination by a vector necessary for fruit formation. In all crossing types, high rates of fruit abortion and formation of seeds without embryos were observed, particularly in interspecific crosses that involvedB. weddellii. Crosses between the more closely relatedB. involutum andB. ipanemense produced fruit formation rates and seed viability similar to those obtained in intraspecific crosses. These results agree with current hypotheses that suggest that interspecific crossing rates reflect phylogenetic proximity. Examination of pollen tube growth and aborted fruits suggests that a series of factors may be involved in the high abortion rate for fruits and large proportion of seeds without embryos. The low interspecific fertility noted forB. weddellii andB. involutum is important in the maintenance of isolation between these species that are sympatric, flower in the same period and share the same pollinators.     

Intraspecific variability in response to stimuli for male and ephippia formation in Daphnia pulex

Natural clones and clones from laboratory crossings of Daphnia pulex have been tested in their response to stimuli for male and ephippia formation. The clones are from ponds in Illinois, USA, an area with both obligate and cyclical clones of D. pulex. The progeny from two types of crosses were studied: 1) crosses between an obligate clone producing males and a cyclical clone producing only females, and 2) self-fertilization within a cyclical clone producing both sexes. Both the natural and the artificial clones showed great variation in response to environmental stimuli for males. Ephippia response seems more equal among the clones, but two of the artificial clones could hardly be stimulated to produce ephippia in the induction experiments. In crowded cultures, however, they produced a few. Crossings between cyclicals and obligates yielded mostly cyclical progeny. Progeny from the selfed clone had low survival and fecundity.     

The influence of habitat heterogeneity on host-pathogen population dynamics

The influence of spatial heterogeneity on the population dynamics of a naturally occurring invertebrate host-pathogen system was experimentally investigated. At ten week intervals over a two year period, I quantified the spatial distribution of natural populations of the terrestrial isopod crustacean Porcellio scaber infected with the isopod iridescent virus (IIV). During the seasonally dry periods of summer and early fall in central California, isopod populations were highly aggregated and the degree of patchiness and distance between inhabited patches was greatest. Coincident with increased patchiness and patch spacing was an increase in isopod density within patches. During the wet seasons of winter and spring, isopod population patchiness, inter-patch spacing, and within-patch density was low. Seasonal changes in virus prevalence were negatively correlated with within-patch density, patchiness, and inter-patch spacing. The influence of the spatial distribution of isopods on virus prevalence was also tested in field experiments. The virus was introduced into arrays of artificial habitat patches colonized by isopods in which interpatch distance was varied. The prevalence of resulting infections was monitored at weekly intervals. In addition, dispersal rates between artificial patches and natural patches were quantified and compared. The results showed that isopods in treatments with the smallest inter-patch spacing had the highest virus prevalence, with generally lower prevalence among isopods in more widely spaced patches. The spacing of experimental patches significantly affected virus prevalence, although the experiments did not resolve a clear relationship between patch spacing and virus prevalence. Rates of dispersal between patches decreased with increased patch spacing, and these rates did not differ significantly from dispersal between natural patches. The results suggest that rates of dispersal between isopod subpopulations may be an important component of the infection dynamics in this system. I discuss the consequences of these findings for host-pathogen dynamics in fragmented habitats, and for other ecological interactions in spatially heterogeneous habitats.     

Search orientation in adultDrosophila melanogaster: Responses of rovers and sitters to resource dispersion in a food patch

Drosophila melanogasteradults were employed in single resource patches of varying density and size and in a multiple-patch array to determine the degree to which resource dispersion influences searching success. Individuals from rover and sitter selected lines, with extreme genotypes for local search duration, are not as successful as control-line (wild-type) flies in locating sucrose drops in single patches varying in size and density. The number of new drops located differed significantly between fly lines in all patch types, except in a high-density patch, and within each fly line over the different patch sizes and densities. The similarities in number of drops found by rovers and sitters in all patch types are not reflected in the time periods spent searching. In the multiple-patch array sitters never left the central patch, whereas most rovers and con-trol-line flies found additional patches. The proximate explanations for the success or failure of the three fly lines in different patch sizes and densities relate to the looping locomotor pattern characterizing local search in D. melanogaster.The reactivation of searching each time a drop is ingested or revisited keeps an individual in the immediate vicinity of the last encountered resource. Flies from the selected lines, each exhibiting extreme types of locomotor patterns, leave patches relatively unexploited because local search consists either of rapid, nearly linear movement away from a drop in rovers or of relatively long bouts of local search in sitters, which promotes revisiting rather than locating new drops. Control-line flies locate more drops than either rovers or sitters and in less time than sitters, suggesting that their intermediate phenotype for search behavior allows for more flexibility in searching in various patch sizes and resource densities. The results are discussed with reference to environmental and physiological factors that may modify searching behavior and, possibly, enhance the survival of individuals with extreme genotypes.     

Relative importance of host plant patch geometry and habitat quality on the patterns of occupancy,extinction and density of the monophagous butterfly <Emphasis Type="Italic">Iolana iolas</Emphasis>

Habitat fragmentation is a major cause of species rarity and decline because it increases local population extinctions and reduces recolonisation rates of remnant patches. Although two major patch characteristics (area and connectivity) have been used to predict distribution patterns in fragmented landscapes, other factors can affect the occurrence of a species as well as the probability of it becoming extinct. In this paper, we study the spatial structure and dynamics of the butterfly Iolana iolas in a 75-patch network of its host plant (Colutea hispanica) to determine the relative importance of patch area, connectivity and habitat quality characteristics on occupancy, extinction and density over the period 2003–2006. Occupancy in 2003, incidence (proportion of years occupied) and probability of extinction were mostly affected by patch area. Smaller patches were less likely to be occupied because they had a higher probability of extinction, partly due to environmental stochasticity. The density of I. iolas was negatively related to patch area in all study years. Only in 2004 was the density of I. iolas positively influenced by fruit production per plant. Our results suggest that for I. iolas, and probably for other specialist butterflies with clearly delimited resource requirements, metapopulation dynamics can be satisfactorily predicted using only geometric variables because most habitat characteristics are subsumed in patch area. However, this hypothesis should be subject to further testing under diverse environmental conditions to evaluate the extent of its generalisation.     

Interspecific interference between Apoanagyrus lopezi and A. diversicornis, parasitoids of the cassava mealybug Phenacoccus manihoti

The parasitoids Apoanagyrus lopezi De Santis and A. diversicornis (Howard) (Hymenoptera: Encyrtidae) have been introduced into Africa for the biological control of the cassava mealybug Phenacoccus manihoti Matile-Ferrero (Homoptera: Pseudococcidae). We have studied competition between these species to investigate if they can coexist. Here we report on the influence of the simultaneous presence of non-conspecific adult females on searching efficiency on patches. Wasps of either species foraged on discs of cassava leaf with mealybugs, while at the same time different numbers of non-conspecifics were also depleting the patch. Patch area per parasitoid and number of hosts available to each parasitoid were equal in all treatments.In both species, the presence of other foragers clearly affected several aspects of the parasitoids' behaviour. Patch residence time increased with the number of non-conspecifics in A. diversicornis. In both parasitoid species, the proportion of hosts left unparasitized after the patch visit decreased with increasing numbers of females on the patch. The proportions of super- and multiparasitism did not change with the number of females. Both species produced more offspring during a patch visit in the presence of more non-conspecifics. These behavioural changes did not, however, lead to a change in the offspring production rate on patches. A. diversicornis produced offspring at a rate three times that of A. lopezi when one A. lopezi and one A. diversicornis foraged simultaneously. This is the first report of an aspect of interspecific competition where A. diversicornis has an advantage over A. lopezi. Interference between adult females thus promotes coexistence of the two species on P. manihoti.