Genetic diversity in the cycad Macrozamia riedlei

Zamia (Cycadales: Zamiaceae) on Puerto Rico: asymmetric genetic differentiation and the hypothesis of multiple introductions

PREMISE OF THE STUDY

This study of Zamia in Puerto Rico is the most intensive population genetics investigation of a cycad to date in terms of number of markers, and one of few microsatellite DNA studies of plants from the highly critical Caribbean biodiversity hotspot. Three distinctive Zamia taxa occur on the island: Z. erosa on the north coast, and Z. portoricensis and Z. pumila, both in the south. Their relationships are largely unknown. We tested three hypotheses about their genetic diversity, including the possibility of multiple introductions.

METHODS

We used 31 microsatellite loci across 10 populations and analyzed the data with AMOVA, Bayesian clustering, and ABC coalescent modeling.

KEY RESULTS

Puerto Rican zamias exhibit an amalgam of patterns of genetic differentiation that have been reported for cycads. Overall, the taxa are slightly inbred, with high infra-populational variation and little evidence of recent bottlenecks. Zamia erosa exhibits a more than threefold greater degree of population differentiation than the other two taxa. Admixture is evident only between Z. portoricensis and Z. pumila. Zamia portoricensis is inferred to be the youngest taxon on the island, on the basis of estimates of coalescence time and effective population size. A selective sweep may be underway in a small population of Z. erosa in a saline environment.

CONCLUSIONS

Zamia erosa may represent an independent introduction into Puerto Rico; Z. portoricensis and Z. pumila fit a scenario of allopatric speciation. This will be explored further in the context of genetic analysis across the entire Caribbean region.

Allozyme variation in the three extant populations of the narrowly endemic cycad Dioon angustifolium Miq. (Zamiaceae) from North-eastern Mexico

BACKGROUND AND AIMS

Dioon angustifolium was considered within D. edule. Recent morphometric and allozyme studies on D. edule have shown that D. angustifolium has originated from geographic isolation and is therefore considered to be a separate species. This cycad is endemic to north-eastern Mexico and is known only from three populations in the Sierra Madre Oriental mountain chain. Its populations are small when compared with its southern relative D. edule. In this study, genetic variation was determined within and between populations of D. angustifolium and the genetic consequences of habitat fragmentation and isolation of populations of this species were assessed.

METHODS

Allozyme electrophoresis of 14 presumptive loci was used. The data were analysed with statistical approximations for estimating genetic diversity, structure, gene flow and recent genetic bottlenecks.

KEY RESULTS

Means and standard deviations of genetic diversity estimators were: number of alleles per locus (A = 1.67 +/- 0.23), percentage of polymorphic loci (P = 52.4 +/- 23 %) and expected heterozygosity (H(E) = 0.218 +/- 0.093). The genetic variation attributable to differences among populations was 16.7 %. Mean gene flow between paired populations was Nm = 1.55 +/- 0.67, which is similar to that reported for endemic plant species of narrow geographical distribution and species with gravity-dispersed seed. A recent bottleneck is detected in the populations studied.

CONCLUSIONS

Dioon angustifolium presents high levels of genetic diversity compared with other cycad species, in spite of small population sizes. The recent bottleneck effect did not effectively reduce the genetic variation to the extent of eliminating these populations. The distribution of D. angustifolium appears to be the result of historical biogeographical effects related to the Pleistocene glaciations. It is recommended that this species be catalogued in the IUCN Red List of Threatened Species and conservation efforts be made to preserve it.

Organelle DNA phylogeography of Cycas taitungensis, a relict species in Taiwan

The phylogegraphic pattern of Cycas taitungensis, an endemic species with two remaining populations in Taiwan, was investigated based on genetic variability and phylogeny of the atpB-rbcL noncoding spacer of chloroplast DNA (cpDNA) and the ribosomal DNA (rDNA) internal transcribed spacer (ITS) of mitochondrial DNA (mtDNA). High levels of genetic variation at both organelle loci, due to frequent intramolecular recombination, and low levels of genetic differentiation were detected in the relict gymnosperm. The apportionment of genetic variation within and between populations agreed with a migrant-pool model, which describes a migratory pattern with colonists recruited from a random sample of earlier existing populations. Phylogenies obtained from cpDNA and mtDNA were discordant according to neighbour-joining analyses. In total four chlorotypes (clades I-IV) and five mitotypes (clades A-E) were identified based on minimum spanning networks of each locus. Significant linkage disequilibrium in mitotype-chlorotype associations excluded the possibility of the recurrent homoplasious mutations as the major force causing phylogenetic inconsistency. The most abundant chlorotype I was associated with all mitotypes and the most abundant mitotype C with all chlorotypes; no combinations of rare mitotypes with rare chlorotypes were found. According to nested clade analyses, such nonrandom associations may be ascribed to relative ages among alleles associated with the geological history through which cycads evolved. Nested in networks as interior nodes coupled with wide geographical distribution, the most dominant cytotypes of CI and EI may represent ancestral haplotypes of C. taitungensis with a possible long existence prior to the Pleistocene glacial maximum. In contrast, rare chlorotypes and mitotypes with restricted and patchy distribution may have relatively recent origins. Newly evolved genetic elements of mtDNA, with a low frequency, were likely to be associated with the dominant chlorotype, and vice versa, resulting in the nonrandom mitotype-chlorotype associations. Paraphyly of CI and EI cytotypes, leading to the low level of genetic differentiation between cycad populations, indicated a short period for isolation, which allowed low possibilities of the attainment of coalescence at polymorphic ancestral alleles.

Allozyme and DNA sequence comparisons of nine species of Encephalartos (Zamiaceae)

Phylogenetic relationships between Encephalartos altensteinii Lehmann, E. arenarius R.A. Dyer, E. horridus (Jacquin) Lehmann, E. latifrons Lehmann, E. lehmannii Lehmann, E. longifolius (Jacquin) Lehmann, E. princeps R.A. Dyer and E. trispinosus (Hooker) R.A. Dyer were studied, using E. ferox Bertoloni f. as outgroup. Three continuous and one discontinuous buffer systems were used and gene products of 14 enzyme coding loci were examined by horizontal starch gel-electrophoresis. Genetic variation was studied in a cultivated population of E. lehmannii and the average heterozygosity value for this population is 13.5%, which falls within the range reported for other cycad species. Fixed allele differences between the species studied was not found at any of the loci studied, which suggest that these species are closely related. DNA sequence analysis of rbcL and ITS 1 & 2 genes (1428 and 895 basepairs, respectively) confirmed the close genetic relationships between these taxa. According to ITS and rbcL sequences E. altensteinii and E. princeps are sibling taxa which form a sister group to E. arenarius, E. horridus, E. latifrons, E. lehmannii, E. longifolius, and E. trispinosus. The genetic distances between both groups were 0.12-0.47% for ITS and 0.08-0.16% for rbcL DNA. The results indicate recent (probably pleistocenic) speciation for this group of cycads, and the relationships are discussed with reference to affinities based on morphology and distribution.

Genetic diversity in the monospecific western australian endemic, geleznowia verrucosa turcz. (Rutaceae)

Allozyme diversity was surveyed at 16 loci across 16 populations of the monospecific, but morphologically diverse, endemic shrub Geleznowia verrucosa Turcz. (Rutaceae). Single-locus diversity measures portrayed G. verrucosa as a genetically depauperate genus (A, 1.4; P, 29.6%), with total genetic diversity (HT, 0.304) partitioned between populations (DST, 0.175), rather than within (HS, 0.129). Some 58% of the total genetic diversity was attributable to interpopulational differences. Although restricted distribution, bottlenecks and/or founder effects and small population size have probably contributed to the low levels of genetic diversity found within this genus, it is evident from this investigation that the genus is not monospecific. The partitioning of genetic diversity, and possible differences in reproductive strategy, suggest that the small- and large-flowered forms represent distinct taxa. Higher levels of divergence between the intermediate-form populations and increased heterozygosity at the Lap locus suggest that this form is of hybrid origin and has arisen through a series of hybrid events between the small and large forms.