A large fraction, sometimes the largest fraction, of a eukaryotic genome consists of repeated DNA sequences. Copy numbers range from several thousand to millions per diploid genome. All classes of repetitive DNA sequences examined to date exhibit apparently general, but little studied, patterns of "concerted evolution." Historically, concerted evolution has been defined as the nonindependent evolution of repetitive DNA sequences, resulting in a sequence similarity of repeating units that is greater within than among species. This intraspecific homogenization of repetitive sequence arrays is said to take place via the poorly understood mechanisms of "molecular drive." The evolutionary population dynamics of molecular drive remains largely unstudied in natural populations, and thus the potential significance of these evolutionary dynamics for population differentiation is unknown. This review attempts to demonstrate the potential importance of the mechanisms responsible for concerted evolution in the differentiation of populations. It contends that any natural grouping that is characterized by reproductive isolation and limited gene flow is capable of exhibiting concerted evolution of repetitive DNA arrays. Such effects are known to occur in protein and RNA-coding repetitive sequences, as well as in so-called "junk DNA," and thus have important implications for the differentiation and discrimination of natural populations.

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Primers for 36 microsatellite loci were developed and employed to characterize genetic stocks and detect possible outcrossing between highly inbred laboratory strains of the self-fertilizing mangrove killifish, Kryptolebias marmoratus. From attempted crosses involving hermaphrodites from particular geographic strains and gonochoristic males from others, 2 among a total of 32 surveyed progenies (6.2%) displayed multilocus heterozygosity clearly indicative of interstrain gametic syngamy. One of these outcross hybrids was allowed to resume self-fertilization, and microsatellite assays of progeny showed that heterozygosity decreased by approximately 50% after one generation, as expected. Although populations of K. marmoratus consist mostly of synchronous hermaphrodites with efficient mechanisms of internal self-fertilization, these laboratory findings experimentally confirm that conspecific males can mediate occasional outcross events and that this process can release extensive genic heterozygosity.

The hermaphroditic Mangrove Killifish, Kryptolebias marmoratus, is the world's only vertebrate that routinely self-fertilizes. As such, highly inbred and presumably isogenic “clonal” lineages of this androdioecious species have long been maintained in several laboratories and used in a wide variety of experiments that require genetically uniform vertebrate specimens. Here we conduct a genetic inventory of essentially all laboratory stocks of the Mangrove Killifish held worldwide. At 32 microsatellite loci, these stocks proved to show extensive interline differentiation as well as some intraline variation, much of which can be attributed to post-origin de novo mutations and/or to the segregation of polymorphisms from wild progenitors. Our genetic findings also document that many of the surveyed laboratory strains are not what they have been labeled, apparently due to the rather frequent mishandling or unintended mixing of various laboratory stocks over the years. Our genetic inventory should help to clarify much of this confusion about the clonal identities and genetic relationships of laboratory lines, and thereby help to rejuvenate interest in K. marmoratus as a reliable vertebrate model for experimental research that requires or can capitalize upon “clonal” replicate specimens.

U.S. GEOLOGICAL SURVEY CIRCULAR 1013Cover. Symbols of the elements discussed in this report are shown. The triangular background represents three-phase partitioning of the elements in aquatic systems the soluble phase, the suspended sediment phase, and the bottom sediment phase.

The watershed of Ward Creek, a tributary to oligotrophic Lake Tahoe in the Sierra Nevada, has been investigated since 1971 with the objective of improving our knowledge of processes of nutrient and sediment release and transport to the lake. Quantitative data on selected stream water parameters were collected for 3 yr (1972—1975) at three stations on Ward Creek, two on the main upper tributaries and one near the stream mouth. Comparative data were collected at a stream mouth station on adjacent Blackwood Creek in the 3rd yr. The parameters were selected on the basis of their significance to eutrophication of Lake Tahoe. Precipitation in a normal year is over 90% snow but annual patterns vary widely and rainfall at any time of year can be highly important in sediment and nutrient transport. Water discharge and the flux of suspended sediments, NO3—N, phosphorus, iron and trace metals were dominated by the spring snowmelt runoff from mid—April to mid—June. However, in 1974 heavy fall and summer rains accounted for a large percentage of the annual flux of sediments and nutrients in a total of only 14 d. The spring runoff was characterized by distinct diel water discharge patterns. Similar but not coincident patterns were found to exist for sediments and nutrients, including NO3—N, but not for soluble phosphorus. The Ward watershed has 87% the area of Blackwood but discharge proportionately much lower quantities of sediment and nutrients in comparable water yields per hectare in water year 1975. This contrast in fluxes was probably accounted for in the history of greater disturbance by man in Blackwood Canyon. The principal source of suspended sediments in Ward Creek was streambank erosion in the lower reaches of the channel. The dominant form of inorganic nitrogen in Ward was NO3—N derived from precipitation, symbiotic nitrogen fixation and nitrification of organic nitrogen in forest soil. Phosphorus and iron were almost entirely in particulate form and thus their periods of flux occurred during high flows and sediment transport. Sediment and nutrient loading of Lake Tahoe from the Ward and Blackwood watersheds reflects a history of soil disturbance and vegetation removal. Logging, fire and stream channel diversion have been the dominant perturbations. Conservative extrapolation of annual loading data from this study to the entire basin indicates that algal nutrient levels in the lake probably have increased sufficiently in the century of man's intensive disturbance of the basin watersheds to account for increased phytoplankton and periphyton production that have been measured and observed since 1958.

We examined spatial, physiological, and morphological niche relationships in a Phoxinus eos-neogaeus gynogenetic complex of cyprinid fish in five drainages influenced by beaver (Castor canadensis) activity along the 294-km 2 Kabetogama Peninsula in Voyageurs National Park of northern Minnesota. Assessment of clonal variation using ''in gel'' DNA fingerprinting of gynogens from three drainages indicated that all drainages contained only a single clone. When environments associated with beaver pond succession were pooled across drainages, P. eos was more abundant in active beaver ponds, while gynogens were at higher frequencies in collapsed pond and stream environments. Gynogen frequency for different successional environments classified according to drainage indicated strong variation among drainages, especially in pond environments, but a consistent pattern of increased gynogen frequency in collapsed ponds or streams. Intensive sampling within one drainage across successional beaver pond sites, physical habitats, and gradients of oxygen concentration indicated that P. eos, P. neogaeus, and gynogens were all most abundant in the highly oxygenated littoral environment of an upland pond. P. eos abundance and its relative frequency in the gynogenetic complex decreased, while gynogen frequency increased, in a poorly oxygenated collapsed beaver pond and beaver meadow. Fish transplant experiments revealed that gynogens survived longer and had greater physiological tolerance to oxygen stress than sexual progenitors, especially P. eos. Principal component analysis of 11 growth-related morphometric characteristics revealed morphological intermediacy of the hybrid gynogens between the sexual progenitors, with factors related to trophic morphology, especially mouth width and mouth length, segregating the different forms. A comparison of morphological relationships in a beaver pond with a gynogen frequency of 25-35% to those of a pond with a gynogen frequency of only 6-7% suggested that there was a release in the trophic morphology of P. eos in response to reduced gynogen abundance. The temporal and spatial unpredictability of environmental conditions in the north temperate landscape inhabited by these fish has apparently resulted in selection for a physiologically flexible, ''general purpose'' clonal genotype, which occupies a broad ecological niche and may make establishment of additional clones in the same ecosystem difficult.

Recombination is unknown in natural popu-