GENETIC CONSEQUENCES OF OUTCROSSING IN THE CLEISTOGAMOUS ANNUAL,IMPATIENS CAPENSIS. II. OUTCROSSING RATES AND GENOTYPIC CORRELATIONS

The genetic consequences of a plant's mating system depend on both the degree of outcrossing and the genetic relationship between mates. We examined the electrophoretic genotypes of seeds derived from cleistogamous (CL) and chasmogamous (CH) flowers in six populations of the facultatively cleistogamous annual, Impatiens capensis. Multilocus estimates of the outcrossing rates for the strongly protandrous CH flowers ranged from 0.29 to 0.71 and were higher than estimates based on single-locus data. Such results suggest that the CH flowers experience variable levels of both geitonogamous self-fertilization and biparental inbreeding. A new and generally applicable technique based on the relative level of inbreeding within progeny groups provided direct estimates of the correlation between the genotypic values of outcrossed mates. These correlations varied widely among populations and contributed up to half of the inbreeding observed among the CH progeny. Such biparental inbreeding biases estimates of the outcrossing rate based on the mixed-mating model downward and influences mating-system evolution by decreasing the “cost of meiosis.”     

POPULATION GENETIC CONSEQUENCES OF DEVELOPMENTAL EVOLUTION IN SEA URCHINS (GENUS HELIOCIDARIS)

Within the sea urchin genus Heliocidaris, changes in early embryonic and larval development have resulted in dramatic differences in the length of time larvae spend in the plankton before settling. The larvae of one species, H. tuberculata, spend several weeks feeding in the plankton before settling and metamorphosing into juveniles. The other species, H. erythrogramma, has modified this extended planktonic larval stage and develops into a juvenile within 3–4 days after fertilization. We used restriction site polymorphisms in mitochondrial DNA to examine the population genetic consequences of these developmental changes. Ten restriction enzymes were used to assay the mitochondrial genome of 29 individuals from 2 localities for H. tuberculata and 62 individuals from 5 localities for H. erythrogramma. Within H. tuberculata, 11 mitochondrial genotypes were identified. A G_ST analysis showed high levels of genetic exchange between populations separated by 1,000 kilometers of open ocean. In contrast, in H. erythrogramma, 13 mitochondrial genotypes differing by up to 2.33% were geographically partitioned over spatial scales ranging from 800 to 3,400 kilometers. Between distant localities, there was complete mitochondrial lineage sorting and large sequence divergence between resulting clades. Over much smaller spatial scales (< 1,000 km), genetic differentiation was due to the differential sorting of very similar genotypes. This pattern of mitochondrial variation suggests that these population differences have arisen recently and may reflect the historical interplay between the restricted dispersal capabilities of H. erythrogramma and the climatic and geological changes associated with Pleistocene Ice Ages.     

ELEMENTAL COMPOSITION,CORRELATIONS, AND RATIOS WITHIN A POPULATION OF STAURASTRUM PLANCTONICUM (ZYGNEMATALES): AN X-RAY MICROANALYTICAL STUDY1

Individual cells of Staurastrum planctonicum (Teil.) within a mixed freshwater phytoplankton sample were analyzed by scanning electron microscope X-ray microanalysis to determine their elemental composition. X-ray emission spectra routinely showed clear peaks of P, S, and Cl, plus monovalent (Na, K, and divalent (Mg, Ca) cations. Si and Cu were also present in lower quantities. Concentrations of individual elements (expressed as mmol.kg⁻¹ dry weight) varied widely among cells, with values over the sample population approximating to a normal distribution. Although intracellular anion and cation levels varied considerably, significant correlations occurred between concentrations of monovalent and divalent cations (mean ratio 1.4), major diffusible anions and cations (mean ratio 1,2), and total levels of electropositive and electronegative elements (mean ratio 1.2). The monovalent cations of K and Na occurred at a mean ratio of 1.2 and were not significantly correlated. Concentrations of individual elements (except Si) showed clear positive correlations within the analyses, with 12 highly significant (99% probability) correlations out of 36 possible combinations. Principal factor analysis showed that elemental correlations were determined by two major factors, with two resulting groups of elements—(Na, S, Cl, Ca, Cu) and (Mg, P, K). Statistical relationships between elements followed a clear correlation pattern, which retained its characteristics even when elemental concentrations were expressed per unit P rather than per unit dry weight. Elemental concentrations were closely similar in matching, but not nonmatching, smicells. The statistical pattern of elemental associations noted in Staurastrum parallels that seen in X-ray micro-analytical studies of other algae but differs in detail. This pattern of statistical associations has biological implications in terms of cell compartmentation, characterization of different cell types, and cell interactions with their environment.     

GENETIC VARIABILITY WITHIN A POPULATION AND BETWEEN DIPLOID/HAPLOID TISSUE OF MACROCYSTIS PYRIFERA (PHAEOPHYCEAE)1

Multi-locus DNA fingerprints using an M13 probe were obtained for eight individuals of giant kelp Macrocystis pyrifera (L.) C. Ag. collected from Monterey Bay, California. For each individual, DNA was extracted from a diploid blade and from ca. 10⁹ haploid spores that were released from four to Jive sporophylls. Viable or swimming spores from one individual were pooled and referred to as a spore group. A total of 34 bands (4–19 kb) was detected in DNA fingerprints from the eight blades and eight spore groups, with individual blade or spore groups exhibiting 7–18 bands (mean = 12.6). One band (4.5 kb) was present in all 16 samples. Eight bands were detected in 11–14 of the 16 samples. Similarity indices were calculated for all pairwise comparisons of fingerprint bands among all possible combinations of blades and spore groups. Mean similarity indices for the eight blades (0.51, SE = 0.032) and spore groups (0.56, SE = 0.031) were significantly lower than for the eight comparisons of the blade and spore groups from a single individual (0.86, SE = 0.052). The data indicate that DNA fingerprints can be used to measure genetic variation within populations of M. pyrifera because variation of DNA fingerprints associated with meiotic products (spores) of a given individual is small relative to variation observed among individuals within the population. Additionally, fingerprint variation between diploid vegetative tissue and haploid meiotic products may be a measure of genetic change due to recombination or DNA turnover mechanisms.     

NATURAL HYBRIDIZATION AND EXTINCTION OF A POPULATION OF PHYSALIS VIRGINIANA (SOLANACEAE)

A population of Physalis virginiana (Solanaceae) in Durham Co., North Carolina, which comprised about 40 individual plants in 1968, was reduced in size to four depauperate plants which were collected and studied in 1973. Concomitant with this reduction in population size is an increase in the proportion of individuals showing evidence of hybridization with the closely related P. heterophylla, also growing in the vicinity, from no evidence of hybridization in 1968 to 75% hybrid individuals (3 out of 4 surviving plants) in 1973. Two possible reasons are postulated for this hybridization: scarcity of P. virginiana pollen for cross-fertilization in this self-incompatible species and superiority of vegetative growth of hybrids in a changing habitat that has become unsuited for the survival of P. virginiana.     

GENETIC STRUCTURE AND DIVERSITY WITHIN LOCAL POPULATIONS OF BACILLUS MYCOIDES

Sixty strains of Bacillus mycoides were isolated from each of two sites and characterized by their responses to standard metabolic tests used in bacterial taxonomy, by multilocus enzyme electrophoresis (MLEE), and by restriction-fragment-length polymorphism (RFLP) analysis of Southern blots probed with both a conserved DNA fragment derived from a Salmonella typhimurium ribosomal cistron and with two cosmid probes derived from B. mycoides ATCC strain 6463. Both MLEE and RFLP analyses indicated that the collection contained two genetically distinct sets of strains (I and II); one of these sets was further differentiated genetically by the same analyses (IIA and IIB). Standard taxonomic analysis did not distinguish these sets of strains; biochemical test profiles were similar for all isolates. The genetic distance between groups I and II is as great as that observed for recognized species of bacteria. It is proposed that these groups are sibling species having a common evolutionary descent and that their metabolic phenotype has been conserved, whereas their DNA and protein sequences have diverged. No strong evidence of geographic differentiation between strains from the two sites appeared in either genetic or phenetic characters.     

CONTROL OF COLONY AND UNICELL FORMATION IN A SYNCHRONIZED SCENEDESMUS12

Colony formation in a synchronized Scenedesmus could be controlled by the addition of 0.05% Na₃ citrate or 85 μM EDTA to modified Bristol's medium. No unicells were formed; only S. quadricauda-like or S. longispina-type colonies were observed in young cultures grown in that medium. A colony population could be made completely unicellular in 2 days if grown in soil-Bristol's medium and transferred daily. The pleomorphic Scenedesmus was examined in synchronized culture. When the organism was grown in a defined medium in a 15-hr light /9-hr dark cycle on a roller tube rotator at 2-6 rpm and transferred daily, synchrony of cell division and release of the products were achieved. In a synchronized culture 2 doublings/day were recorded, with most cytoplasmic cleavages and all releasing of daughter cells taking place in the dark period. In many observations with several synchronized strains of Scenedesmus, no fixed pattern of release of daughter products from mother cells or colonies was detected. Colonies or unicells had their full spine complement at the time of release. As a cell or colony aged the spines sometimes increased in thickness. Other Scenedesmus strains were examined to provide supporting data.     

LIFE-HISTORY VARIATION WITHIN A POPULATION OF THE COLONIAL ASCIDIAN BOTRYLLUS SCHLOSSERI. I. THE GENETIC AND ENVIRONMENTAL CONTROL OF SEASONAL VARIATION

Many empirical analyses of life-history tactics are based on the assumption that demographic variation ought to be greatest among populations or species living in different environments. However, in a single population of the sessile colonial sea squirt Botryllus schlosseri, there are two discrete life-history morphs. Semelparous colonies are characterized by a) death immediately following the production of a single clutch, b) early age at first reproduction, c) rapid growth to first reproduction, and d) high reproductive effort. In contrast, iteroparous colonies a) produce at least three clutches before dying, b) postpone sexual reproduction until they are nearly twice the age of semelparous colonies, c) grow at about half the rate of semelparous colonies, and d) invest roughly 75% less in reproductive effort than semelparous colonies. Semelparous colonies numerically dominate the population through midsummer; later in the summer, iteroparous colonies are most numerous. Field and laboratory common-garden experiments, along with breeding studies, indicate that the demographic differences between the morphs are genetically determined. Consequently, the seasonal switch from dominance by semelparous colonies to dominance by iteroparous colonies may be an evolved response to a seasonally changing environment. On theoretical grounds, temporal variation in selection is thought to play a relatively unimportant role in maintaining genetic polymorphism; nonetheless, the seasonally recurrent life-history polymorphism shown in this study indicates that temporal variation in selection can lead to the maintenance of genetic polymorphism for traits strongly affecting fitness.     

POPULATION DYNAMIC CONSEQUENCES AND EVOLUTION OF SEED TRAITS OF HETEROSPERMA PINNATUM (ASTERACEAE)

We summarize research on variation in achene morphology of Heterosperma pinnatum. Each flowering head has a broad range of achene morphologies. There is no between-year seed bank. There are no ecologically relevant differences in achene size, seedling size, or seedling growth rates of the different morphs. Achenes located centrally in the heads lose dormancy earlier during the period between autumn achene production and the onset (May-July) of the rainy season. This results in relatively more early emergence of central achenes. Awned central achenes also have greater adhesive dispersibility. Emergence, survival, and fecundity were measured for achenes sown into natural populations in central Mexico. When harsh conditions result in few seeds surviving to reproduce, the advantage tends to go to the more “conservative” peripheral achenes. Greenhouse experiments show a strong genetic component to the determination of achene proportions within and among populations and a variety of genetic correlations. Populations with a high proportion of awned achenes tend to be found in vegetation types where they are likely to be ephemeral. A high proportion of central achenes tend to be found in sites with little pre-rainy season precipitation. These patterns are interpreted in terms of selection for dispersal and the within-year timing of germination.     

THE DUAL ROLE OF SELECTION AND EVOLUTIONARY HISTORY AS REFLECTED IN GENETIC CORRELATIONS

The patterns of genetic correlations between a series of eye and antenna characters were compared among two sets of spring-dwelling and cave-dwelling populations of Gammarus minus. The two sets of populations originate from different drainages and represent two separate invasions of cave habitats from surface-dwelling populations. Matrix correlations, using permutation tests, indicated significant correlations both between populations in the same basin and from the same habitat. The technique of biplot, which allows for the simultaneous consideration of relationships between different genetic correlations and different populations, was used to further analyze the correlation structure. A rank-3 biplot indicated that spring and cave populations were largely differentiated by eye-antennal correlations, whereas basins were differentiated by both eye-antennal and antennal-antennal correlations. Eye-antennal correlations, which are likely to be subject to selection, were most similar within habitats, which are likely to have similar selective regimes.