Population Growth in Planaria Dugesia tigrina (Gerard) : Regulation by the absolute number in the population

Planaria reproduce by transverse fission. Isolated worms increase in number exponentially, while social animals at the same density are inhibited in terms of numerical increase, but over a 25 day period undergo a larger increase in mass. Isolated posterior fission products reproduce faster than isolated anterior fission products. Regulation of population growth is independent of density over a 16-fold range and regulatory factors cannot be demonstrated in the medium. Regulation of population growth depends on direct contact between animals. Fission period varies from individual to individual and from period to period for a given individual. Doubling time is related to the absolute number of individuals comprising the population as follows: P_N = (P_M · N)/(K + N), where P_N is the doubling period of a population of N individuals, P_M is the doubling time of an infinitely large population, N is the number of individuals in the population, and K is the number of individuals in a population the period of which is one-half P_M. At 22°–24°C P_M is estimated to be 43.3 days and K is 1.87 individuals. A model system assumes that inhibitor flows through the population from animal to animal from the slowest to the fastest animal in the population thus acting to synchronize population increase as well as to determine the rate of population growth. A possible source of the inhibitor is discussed.     

Chromosome number ofSaruma oliver (Aristolochiaceae)

Chromosome number ofSaruma henryi, the only species of the genus, was counted for the first time. The species has 2n=52 chromosomes, and its chromosomal complement is characteristically composed of very small chromosomes. Based on chromosome data available for the family, comparisons indicate an isolated position ofSaruma in Aristolochiaceae.     

Chromosome number inHalophytum ameghinoi (Halophytaceae)

The chromsome number and karyotype ofHalophytum ameghinoi (Speg.) Speg. was found to be 2n = 24 and 12m+10sm+2t, respectively. Meiosis was regular and 12 bivalents were observed. The relationships of this monotypic family are briefly discussed.     

The responses to current flow of two stream dwelling triclads,Crenobia alpina (Dana) and Polycelis felina (Dalyell)

Summary An apparatus is described which allows triclads to choose between the fast and slow regions of a stirred tank. In the absence of a stone substratum Crenobia alpina preferred or tolerated higher current speeds than Polycelis felina, this effect was enhanced when a stone substratum was added to the apparatus. These results indicate that the absence of Polycelis felina from the steep gradient regions of streams could be accounted for by its preference for slower micro-currents. Conversely the presence of Crenobia alpina in steep gradient regions could be accounted for by its preference for or tolerance to higher micro-currents.     

Purging of inbreeding depression within a population of Oxalis alpina (Oxalidaceae)

? Premise of the study: Variation among individuals in levels of inbreeding depression associated with selfing levels could influence mating system evolution by purging deleterious alleles, but empirical evidence for this association is limited. ? Methods: We investigated the association of family-level inbreeding depression and presumed inbreeding history in a tristylous population of Oxalis alpina (Oxalidaceae). ? Key results: Mid-styled individuals possessed the greatest degree of self-compatibility (SC) and produced more autogamous capsules than short- or long-styled individuals. Offspring of highly self-compatible mid-styled individuals showed reduced inbreeding depression. Mid-styled plants that produced capsules autogamously exhibited reduced stigma-anther separation compared to mid-styled plants that produced no capsules autogamously. Reduced inbreeding depression was not correlated with stigma-anther separation, suggesting that self-compatibility and autogamy evolve before morphological changes in stigma-anther separation. ? Conclusions: Purging of inbreeding depression occurred in SC mid-styled maternal families. Low inbreeding depression in SC mid-styled plants may lead to retention of the mid-styled morph in populations, despite the occurrence of higher selfing rates in mid-styled relative to short- or long-styled morphs. Variation among individuals in levels of self-fertilization within populations may lead to associations between inbreeding lineages and lower levels of inbreeding depression, influencing the evolution of mating systems.     

Population structure of the freshwater flatworm Crenobia alpina (Dana): old lineages and low gene flow

Aim  The freshwater flatworm Crenobia alpina lives almost exclusively in headwaters of mountainous areas and is supposed to be a glacial relict. We examined genetic diversity within and between populations of C. alpina in order to determine the taxonomic status of purported subspecies and to understand large-scale biogeographical patterns of glacial relicts.
Location  Central Europe.
Methods  We analysed mitochondrial DNA sequences and polymorphic allozyme loci of C. alpina populations across its range in central Europe. Sequences were compared using parsimony, minimum evolution and maximum likelihood. Allozymes were analysed using traditional as well as Bayesian estimates of F statistics.
Results  We found considerable divergence between haplotypes. For each of the two lineages occurring throughout central Europe, allozymes showed considerable differentiation between populations and a strong isolation by distance effect. Hence populations are effectively isolated even across rather small spatial scales.
Main conclusions  There is strong evidence that C. alpina is a complex of distinct lineages or cryptic species that date back to the late Miocene. The separation of lineages may be associated with the formation of deep valleys at the end of the Messinian Crisis.     

Chromosome number determinations in Aster section Conyzopsis (Asteraceae)

Chromosome numbers forAster brachyactis, A. frondosus, andA. laurentianus were determined to be 2n=14. The latter two are the first documented records for these taxa. The basic chromosome number for sectionConyzopsis is confirmed asx=7.     

Chromosome number of theOxalis tuberosa alliance (Oxalidaceae)

Twelve taxa of theOxalis tuberosa alliance were analysed and found to share the same basic chromosome number x = 8. The karyotypes are composed by small metacentric and submetacentric chromosomes. Different ploidy levels were found among the taxa: there were 9 diploids, 1 tetraploid, 1 hexaploid and 1 octoploid. The last ploidy level corresponds toO. tuberosa, the only tuber bearing taxon found so far in the alliance. Cytotaxonomic evidence and evolutionary considerations suggest to classify theO. tuberosa alliance in sect.Herrerea.     

Chromosome number evolution in the tribeBrassiceae (Brassicaceae): Evidence from isozyme number

Variation in isozyme number was used to assess the evolution of haploid chromosome numbers (n=6–75) and systematic relationships in the tribeBrassiceae, which is believed to be one of the few monophyletic tribes in theBrassicaceae. Ten enzyme systems were surveyed among 108 species in 35 genera of tribeBrassiceae and for 11 species from seven other tribes. The data indicated that taxa with n=7–13 and n=14–18 were similar in isozyme number, suggesting that genera with n=14–18 did not arise from polyploidy (i.e. entire duplication) of the n=7–13 genomes. These results suggest that aneuploidy and/or chromosome fusion/splitting have played a more significant role than polyploidy in the evolution of higher base chromosome numbers in the tribe. The detection of widespread isozyme duplication in the tribe is consistent with reports of extensive gene duplication in theBrassica crop species, and suggests that the common ancestor of the tribe already had undergone a polyploid event, i.e. complete genome duplication, prior to aneuploid divergence. Inheritance studies conducted onSinapis arvensis showed that segregation ratios at seven loci (Fbp-2,Gpi-2,Idh-2,Pgm-2,Pgm-2,Tpi-1,Tpi-1) conformed to those expected under Mendelian inheritance. Isozyme duplications were phylogenetically informative at various taxonomic levels in the tribe. In particular, duplications for cytosolic phosphoglucomutase (Pgm-2,Pgm-2) and plastid triosephosphate isomerase (Tpi-1,Tpi-1) were evident in 33 of the 35 genera examined, supporting the monophyletic status of theBrassiceae with the inclusion ofOrychophragmus and the exclusion of controversial membersCalepina andConringia.     

Chromosome number and flavonoid synthesis in Briza L. (Gramineae)

Following a recent reidentification of the apigenin C-glycosides in diploid Briza media, the luteolin C-glycosides were reexamined and three acylated derivatives of a luteolin C-glycoside were found. In an attempt to identify the chromosome or group of chromosomes responsible for the change in flavonoid synthesis from 4'-hydroxy- in diploid plants to 3',4'-dihydroxyflavone C-glycosides in autotetraploid plants, leaf flavonoids of artificially produced aneuploids of B. media were examined. Among these plants three different leaf flavonoid profiles were recognized: diploid, tetraploid, and a "modified" tetraploid pattern. All the aneuploids with the normal or "modified" tetraploid pattern were trisomic for one of the small acrocentric chromosomes. Induced polyploids of other Briza species were usually found to have similar flavonoid patterns in the two chromosome races. Flavonoid sulfates were found in three South American species but are absent from all the European species.     

Chromosome number variation and polyploidy in the genus Echinocereus (Cactaceae)

Analyses of meiotic and mitotic chromosomes were undertaken in 16 taxa of Echinocereus belonging to 12 species and all seven taxonomic sections (sensu Taylor). Chromosome numbers are reported for the first time for eight taxa, and previously published chromosome counts are confirmed for the remaining eight. Both diploid and polyploid counts were obtained. Eleven (69%) of the taxa surveyed were diploid (2n = 22); the five varieties of E. engelmannii were polyploid (2n = 44). Overall, chromosome counts are available for 23 of the 48 proposed species (sensu Taylor). Of these, 19 (82%) are diploid, and four (18%) are polyploid. Polyploid cytotypes are most common in the primitive sections, e.g., sections Erecti and Triglochidiatus, which suggests that polyploidy is probably a derived condition in Echinocereus. Polyploid taxa range from medium to high latitudes and elevations relative to the overall distribution of the genus. Polyploidy, hybridization, and cryptic chromosomal rearrangements are thought to be the major causes of the speciation events of the genus.     

Chromosome number in the lizard genus Uta (family Iguanidae)

A chromosome number of 34 (12 macro- and 22 microchromosomes) was found to be characteristic of the bone marrow in 47 animals including males from the species Uta antiquus, and both males and females from the following species and subspecies: Uta stansburiana stansburiana, Uta stansburiana stejnegeri, Uta stansburiana elegans, Uta stansburiana klauberi, Uta stansburiana mannophorus, Uta nolascensis, Uta palmeri, and Uta squamata. — Diploid chromosome numbers of 34 and haploid numbers of 17 were found in the nine testis smears examined. — The presence of a large number of hypodiploid figures in the bone marrow smears is attributed to cell fragmentation and the problem of distinguishing the small microchromosomes. — Series of polyploid figures whose chromosome numbers increased in arithmetic rather than geometric progressions were observed in the testis dry smears. Possible alternatives for the origin of these figures are presented. — Problems encountered in the use of chromosome number as a taxonomic character are discussed.Supported in part by Research Grants GB-366 and GB-5416 from the National Science Foundation, and GM-15361 from the United States Public Health Service.     

Chromosome number and genome size variation in Colocasia (Araceae) from China

Journal of Plant Research - Chromosome number and genome size are important cytological characters that significantly influence various organismal traits. We investigated chromosome number and...     

Chromosome studies in a neonatal population

The results of chromosome studies on 6809 consecutive newborn infants are presented. One hundred and one (1.48%) were heterozygous for a marker chromosome, the significance of which is not at present clear. Twenty-two infants (0.32%) had a major chromosome abnormality. Only six of these infants (0.09%) had a clinically recognizable abnormal phenotype (Down''s syndrome). The occult chromosome abnormalities included five sex chromosome abnormalities (one 47,XYY; two 47,XXY; two 47,XXX) and 11 balanced translocations. Seven of these were t(DqDq) and four were reciprocal translocations. The results of the present survey are combined with four other similar neonatal surveys in which a total of 23,328 newborns have been screened. Of these, 117 (0.5%; range 0.65-0.32%) had major chromosome abnormalities. The majority of these (72.7%) would not have been detected at birth without chromosome studies, an important fact in the context of prenatal diagnosis of chromosome disease and the early ascertainment of high-risk families.