Bryophytes are thought to be unique among land plants in lacking the important evolutionary process of allopolyploidy, which involves interspecific hybridization and chromosome doubling. Electrophoretic data show, however, that the polyploid moss Plagiomnium medium is an aflopolyploid derivative of Plagiomnium eUipticum and Plagiomnium insigne, that P. medium has originated more than once from these progenitors, and that cross-fertilization results in interlocus genetic recombination. Evidence from restriction fragment length polymorphisms in chloroplast DNA implicates P. insigne as the female parent in interspecific hybridizations with P. ellipticum. Contrary to prevailing views, it appears that those evolutionary processes responsible for genetic differentiation and speciation in other land plants occur in the bryophytes as well.It is estimated that 47% of all flowering plants and 95% of all pteridophytes are polyploids and that the majority of these are allopolyploids, which have arisen through doubling of chromosomes in interspecific hybrids (ref. 1, but see ref.2). Among the bryophytes 79% of mosses, 11% of liverworts, and 2% of hornworts are believed to be polyploids (3, 4). In contrast to other land plants, however, all of these have been assumed on the basis of morphology to be autopolyploids (5, 6). Furthermore, hybridization has been documented in only a few isolated cases involving sterile sporophytes from intergeneric crosses (3-6). The absence of hybrid speciation, an important mode of evolution in other land plants, has contributed to the characterization of bryophytes as a group with severely limited evolutionary potential (7,8).Within the moss genus Plagiomnium, section Rosulata includes one polyploid species (Plagiomnium medium; n = 12) and six haploid species (9). On the basis of morphology, Lowry (10) concluded that Plagiomnium ellipticum (n = 6) is the progenitor of autopolyploid P. medium. Koponen (9), however, stressed different characters and argued that Plagiomnium insigne (n = 6) is the only possible progenitor. Because the morphological evidence is ambiguous, we elected to use biochemical evidence to resolve the origin of P. medium. MATERIALS AND METHODSFour gametophytic populations each of P. medium and P. ellipticum were sampled in northern Michigan from many of the same localities sampled in Lowry's (9) original cytological study. Because Lowry's (9) chromosome counts were done on plants collected in Michigan [all vouchers verified by Wyatt (11)] and his and all other counts [reviewed by Koponen (12)] for these species are in agreement, we did not deem it necessary to examine our material cytologically. We also obtained population samples of P. insigne from northern California. From each population, we collected 36 moss clumps (approximately 5 x 5 cm). These were maintained in a growth chamber on a light/dark cycle of 16:8 hr at 15'C and 10'C, respectively. Single healthy shoots were subjected to horizontal starch gel electrophoresis after 1-2 months under these conditions. E...

No abstract

No abstract

Horizontal starch-gel electrophoresis was used to measure variability at 14 enzyme loci from 13 natural populations of the dioecious moss Plagiomnium ciliare. Overall levels of genetic polymorphism were unexpectedly high for a haploid organism. Using a 1% frequency criterion, 71% of the loci surveyed were polymorphic for the species as a whole. The number of alleles per polymorphic locus for the species as a whole was 2.82 ± 0.34 (mean ± standard error), and mean gene diversity per locus was 0.078 ± 0.035. While total gene diversity (H = 0.178) was similar to that observed for highly outcrossed diploid plants such as pines, the variance within (H = 0.098 ± 0.027) and among (D = 0.080 ± 0.033) populations was more evenly distributed than that reported for populations of conifers. Genetic distances between populations ranged from 0.0002 to 0.2064, with mosses from the Piedmont region of the southeastern United States showing less differentiation among populations than did mosses from the Appalachian Mountains. Gene diversity was much reduced in populations from disturbed, secondary forests in the Piedmont (0.058 ± 0.018) relative to those from minimally disturbed, primary forests in the mountains (0.146 ± 0.048). Intensive sampling within populations revealed heterogeneity even within small (5 × 5 cm) clumps. The discovery of high levels of genetic variability in a plant with a dominant haploid life cycle challenges the traditional view of bryophytes as a genetically depauperate group. Multipleniche selection is proposed as a possible explanation for this anomaly, but the data are also consistent with the view that allozyme polymorphisms are selectively neutral.

No abstract