Positive selection driving the evolution of a gene of male reproduction, Acp26Aa, of Drosophila: II. Divergence versus polymorphism

The evolution of the gene for a male ejaculatory protein, Acp26Aa, has been shown to be driven by positive selection when nonsibling species in the Drosophila melanogaster subgroup are compared. To know if selection has been operating in the recent past and to understand the details of its dynamics, we obtained DNA sequences of Acp26Aa and the nearby Acp26Ab gene from 39 D. melanogaster chromosomes. Together with the 10 published sequences, we analyzed 49 sequences from five populations in four continents. The southern African population is somewhat differentiated from all other populations, but its nucleotide diversity is lower at these two loci. We find the following results for Acp26Aa: (1) The R: S (replacement : silent changes) ratio is significantly higher in the between-species comparisons than in the within-species data by the McDonald and Kreitman test. Positive selection is probably responsible for the excess of amino acid replacements between species. (2) However, within-species nucleotide diversity is high. Neither the Tajima test nor the Fu and Li test indicates a reduction in nucleotide diversity due to positive selection in the recent past. (3) The newly derived nucleotides in D. melanogaster are at high frequency significantly more often than predicted by the neutral equilibrium. Since the nearby Acp26Ab gene does not show these patterns, these observations cannot be attributed to the characteristics of this chromosomal region. We suggest that positive selection is active, but may be weak, for each amino acid change in the Acp26Aa gene.      

Effect of pollination timing on the rate of apomictic reproduction revealed by RAPD markers in paspalum notatum

Progeny tests employing molecular markers allow the identification of individuals originated by sexual means among the offspring of a facultative apomict. The objective of this work was to evaluate the effect of the pollination timing on the proportion of sexually formed individuals in progenies of a facultative apomictic Paspalum notatum genotype. Progeny families of approx. 30 plants each were generated at five different pollination times: 1-3 d pre-anthesis; at anthesis; and 2, 4 and 6 d post-anthesis. Cytoembryological analyses indicated that approx. 17% of the ovules carried a meiotic cytologically reduced embryo sac in florets formed simultaneously with those used for crosses. The parental plants and the five F1 families were analysed using RAPD molecular markers. Ninety-five oligonucleotides were assayed on the progenitors in order to search for male-specific bands. Eight primers presenting clear polymorphic bands were selected for use in the progeny tests. The proportion of sexually produced progeny reached 3.4% before anthesis and 20 % at anthesis, while pollination after anthesis generated only maternal plants. A second progeny of 97 plants obtained from pollination at anthesis produced 16 off-type plants (16.5%), of which only one was a B(III) hybrid (2n + n). Our results indicate that pollination at anthesis allows the greatest potential for sexuality to be expressed in this facultative apomictic genotype. When pollination is delayed as soon as 2 d after anthesis, only the aposporous sacs develop endosperm through pseudogamy to set seed.     

Genetic diversity in sexual diploid and apomictic tetraploid populations of Paspalum notatum situated in sympatry or allopatry

Paspalum notatum is a subtropical grass widely distributed in the temperate areas of America. Diploids are sexual while polyploids give rise to clonal seeds through aposporous apomixis. RAPD markers were used to analyze the genetic structure of three natural populations: i) diploids reproducing sexually (R2X); ii) sympatric apomictic tetraploids collected in the vicinity of the diploids (R4X); iii) allopatric apomictic tetraploids growing in isolation (C4X). The apomictic reproduction rate was evaluated by the use of molecular markers in progeny tests, while chromosome-counting allowed the verification of ploidy levels. Data revealed that the R4X group presented a variation considerably higher than that observed for C4X. Jaccards coefficients were used to produce a cluster diagram using the UPGMA method. All but one tetraploid genotypes grouped together and were associated to diploid genotype A21. The possibility of occasional generation of novel tetraploid clones from the interaction between tetraploid and diploid individuals is discussed.     

Ohio USA stoneflies (Insecta, Plecoptera): species richness estimation, distribution of functional niche traits, drainage affiliations, and relationships to other states

Ohio is an eastern USA state that historically was >70% covered in upland and mixed coniferous forest; about 60% of it glaciated by the Wisconsinan glacial episode. Its stonefly fauna has been studied in piecemeal fashion until now. The assemblage of Ohio stoneflies was assessed from over 4,000 records accumulated from 18 institutions, new collections, and trusted literature sources. Species richness totaled 102 with estimators Chao2 and ICE Mean predicting 105.6 and 106.4, respectively. Singletons and doubletons totaled 18 species. All North American families were represented with Perlidae accounted for the highest number of species at 34. The family Peltoperlidae contributed a single species. Most species had univoltine-fast life cycles with the vast majority emerging in summer, although there was a significant component of winter stoneflies. Nine United States Geological Survey hierarchical drainage units level 6 (HUC6) were used to stratify specimen data. Species richness was significantly related to the number of unique HUC6 locations, but there was no relationship with HUC6 drainage area. A nonparametric multidimensional scaling analysis found that larger HUC6s in the western part of the state had similar assemblages with lower species richness that were found to align with more savanna and wetland habitat. Other drainages having richer assemblages were aligned with upland deciduous and mixed coniferous forests of the east and south where slopes were higher. The Ohio assemblage was most similar to the well-studied fauna of Indiana (88 spp.) and Kentucky (108 spp.), two neighboring states. Many rare species and several high quality stream reaches should be considered for greater protection.     

Genetic response of Paspalum plicatulum to genome duplication

Paspalum plicatulum is a perennial rhizomatous grass with natural diploid and polyploid cytotypes. In this study, we investigated the occurrence of sequence polymorphisms arising immediately after genome autoduplication in this species. Two mixoploid plants (4C and 7D) were previously obtained through colchicine treatment of seeds generated by open pollination of a diploid plant (H14-2x). Diploid and tetraploid sectors from both mixoploids were dissected to generate two ploidy series (4C-2x/4C-4x and 7D-2x/7D-4x). Molecular fingerprints were generated from the maternal plant H14-2x, both ploidy series (4C-2x/4C-4x and 7D-2x/7D-4x), and a tetraploid plant (C1) produced by selfing 7D-4x. Our results indicate that immediately after polyploidization P. plicatulum suffers genetic rearrangements affecting ~28–38 % of the genome. Band gain and loss were equally prevalent at a statistically significant level. At least 5.62 % of the genome experimented recurrent genetic variation in a non-random basis with a confidence of 94.88 %. A significant proportion of novel bands (36 out of 195; 18.4 %) was detected in the C1 tetraploid plant. Half of these bands were not amplified in either H14-2x or 7D-4x, while the remainders were present in H14-2x but absent in 7D-4x. Our results indicate the occurrence of a considerable number of genetic changes in P. plicatulum immediately after polyploidization, some of which were recurrently detected in different independent events. Moreover, we confirmed that after polyploidization, lost ancestral alleles were spontaneously recovered in further generations, a phenomenon previously reported by other research groups.     

Genbank accession #: AF 135190

Plant Molecular Biology -