GENETIC DIFFERENTIATION,SYMPATRIC SPECIATION,AND THE ORIGIN OF A DIPLOID SPECIES OF STEPHANOMERIA

Evidence is presented that a geographically peripheral population of the annual Stephanomcria exigua ssp. coronaria (Compositae), a widespread and ecologically diverse species, has recently given rise by a process of sympatric speciation to a diploid species presently designated “Malheurensis.” The new species comprises less than 250 individuals and is found only at a single locality in eastern Oregon where it grows interspersed with its parental population. Stephanomeria exigua ssp. coronaria is an obligate outcrosser and “Malheurensis” is highly self-pollinating. Reproductive isolation is maintained by differences in breeding system, a crossability barrier that reduces seed set following cross-pollination between them, and reduction in hybrid fertility caused by chromosomal structural differences. They are very similar morphologically. Electrophoretic analyses of seven enzyme systems demonstrate that all the alleles but one at the controlling 13 gene loci in “Malheurensis” are identical to alleles in ssp. coronaria. The new species displays certain maladapted features including loss of the specific requirements for seed germination characteristic of the progenitor population of ssp. coronaria. The origin of “Malheurensis” appears to be an exception to the theory of geographical speciation because spatial isolation is not necessary at any time for the origin or establishment of its reproductive isolating barriers. The nature of these barriers plus the genetic homogeneity of the species are compatible with the hypothesis that it derives from a single progenitor individual. Very little genetic change is involved initially in this mode of speciation.     

PLOIDAL CHANGES IN CLONAL CULTURES OF SPIROGYRA COMMUNIS AND IMPLICATIONS FOR SPECIES DEFINITION

A clonal culture of Spirogyra filaments of initially uniform width produced filaments of three additional significantly different widths. Group I filaments of the original clone were 30.9 ± 0.7 μm wide (mean ± SD, N = 50). Group I filaments produced Group II filaments (22.0 ± 1.1 μm) through vegetative growth and sexual reproduction. Zygospores from homothallic Group I filaments produced germlings representative of Groups I and II; zygospores from homothallic Group II filaments produced germlings representative of Group II only. Germlings of Groups III (27.7 ± 1.0 μm) and IV (44.9 ± 0.8 μm) were produced in the cross of I × II. Viable zygospores from homothallic Group III filaments were obtained. Cells of Group IV filaments were initially binucleate and did not conjugate. Of the six intergroup crosses possible, four resulted in conjugation-tube formation only; two crosses yielded zygospores (I × II and III × IV). Germlings from the successful cross of Groups III and IV produced filaments of all four groups. Chromosome counts were: Group I (24), Group II (12), Group III (18), and Group IV (24, one nucleus). Relative nuclear fluorescence values of mithramycin-stained DNA were (mean ± SD, N ≥ 30): Group I (11.1 ± 1.4), Group II (5.7 ± 0.7), Group III (8.8 ± 1.3), and Group IV (10.0 ± 0.9, one nucleus). Cytologically, Group II appears to be a diploid (2x), Group I a tetraploid (4x), and Group III a triploid (3x). Systematically, Groups I, II, and III key out to Spirogyra singularis, S. communis, and S. fragilis, respectively, using Transeau's mongraph of the family Zygnemataceae. These species are interpreted to represent a species complex of S. communis (whose name has priority) with the ancestral haploid (x = 6) missing.     

FEMALE HETEROGAMETY AND SPECIATION: REDUCED INTROGRESSION OF THE Z CHROMOSOME BETWEEN TWO SPECIES OF NIGHTINGALES

Several lines of evidence suggest that the X chromosome plays a large role in intrinsic postzygotic isolation. The role of the Z chromosome in speciation is much less understood. To explore the role of the Z chromosome in reproductive isolation, we studied nucleotide variation in two closely related bird species, the Thrush Nightingale ( Luscinia luscinia ) and the Common Nightingale ( L. megarhynchos ). These species are isolated by incomplete prezygotic isolation and female hybrid sterility. We sequenced introns of four Z-linked and eight autosomal loci and analyzed patterns of polymorphism and divergence using a divergence-with-gene flow framework. Our results suggest that the nightingale species diverged approximately 1.8 Mya. We found strong evidence of gene flow after divergence in both directions, although more introgression occurred from L. megarhynchos into L. luscinia . Gene flow was significantly higher on the autosomes than on the Z chromosome. Our results support the idea that the Z chromosome plays an important role in intrinsic postzygotic isolation in birds, although it may also contribute to the evolution of prezygotic isolation through sexual selection. This highlights the similarities in the genetic basis of reproductive isolation between organisms with heterogametic males and organisms with heterogametic females during the early stages of speciation.     

THE SPECIATION HISTORY OF THE PHYSCOMITRIUM—PHYSCOMITRELLA SPECIES COMPLEX

A central problem in evolutionary biology is identifying factors that promote the evolution of reproductive isolation. Among mosses, biogeographic evidence indicates that the potential for migration is great, suggesting that biological factors other than geographic isolation may be critical for speciation in this group. The moss Physcomitrella patens (Funariaceae) has long been used as a model for interspecies hybridization and has recently emerged as an important model system for comparative genomics. We report genealogical analyses of six loci from several populations of P. patens and related species in the genus Physcomitrium. These results unambiguously indicate that the so-called genus Physcomitrella arose at least three times from distinct ancestors within the genus Physcomitrium. In spite of the evidence for natural hybridization in the Physcomitrella–Physcomitrium complex, genealogical and experimental hybridization data indicate that the taxonomically defined species are reproductively isolated. However, these analyses suggest that Physcomitrium eurystomum was formed from a hybridization event between two early diverging lineages in the complex, and that the ancestral population size of these lineages was much smaller than the current population sizes. We discuss these findings in the context of the inferred mating system in the Physcomitrella–Physcomitrium complex and patterns of speciation and diversification.