Into Africa: Molecular phylogenetics and historical biogeography of sub-Saharan African woodferns (Dryopteris)

PREMISE OF THE STUDY

Our goal was to infer the phylogenetic relationships and historical biogeography of the genus Dryopteris with a focus on taxa in sub-Saharan Africa and neighboring islands. In general, little is known about the relationships between African fern species and their congeners in other geographic regions, and our aim was to determine whether the sub-Saharan African species of Dryopteris are monophyletic and evolved within Africa or arrived there via repeated dispersals into Africa from other regions.

METHODS

We obtained sequence data for five chloroplast markers from 214 species of Dryopteris and 18 outgroups. We performed phylogenetic and molecular dating analyses using a Bayesian relaxed clock method in BEAST with fossil and secondary calibration points and estimated ancestral ranges for the genus globally by comparing multiple models in BioGeoBEARS.

KEY RESULTS

We found that 22 of 27 accessions of sub-Saharan African Dryopteris belong to a large clade of 31 accessions that also includes taxa from Indian and Atlantic Ocean islands. Additional accessions of taxa from our regions of interest have Asian, Hawaiian, European, or North American species as their closest relatives.

CONCLUSIONS

The majority of sub-Saharan African Dryopteris species are descended from a shared common ancestor that dispersed to Africa from Asia approximately 10 Ma. There have been subsequent dispersal events from the African mainland to islands in the Atlantic and Indian Oceans, including Madagascar. Several additional species are estimated to have descended from ancestors that reached Africa via separate events over the last roughly 20 million years.

Between sexual and apomictic: unexpectedly variable sporogenesis and production of viable polyhaploids in the pentaploid fern of the Dryopteris affinis agg. (Dryopteridaceae)

BACKGROUND AND AIMS

In ferns, apomixis is an important mode of asexual reproduction. Although the mechanisms of fern reproduction have been studied thoroughly, most previous work has focused on cases in which ferns reproduce either exclusively sexually or exclusively asexually. Reproduction of ferns with potentially mixed systems and inheritance of apomixis remains largely unknown. This study addresses reproduction of the pentaploid Dryopteris × critica, a hybrid of triploid apomictic D. borreri and tetraploid sexual D. filix-mas.

METHODS

Spore size, abortion percentage and number of spores per sporangium were examined in pentaploid plants of D. × critica grown in an experimental garden. The sporangial content of leaf segments was cultivated on an agar medium, and DNA ploidy levels were estimated by DAPI flow cytometry in 259 gametophytes or sporophytes arising from the F2 generation of the pentaploid hybrid.

KEY RESULTS

The hybrid is partly fertile (89-94% of aborted spores) and shows unstable sporogenesis with sexual and apomictic reproduction combined. The number of spores per sporangium varied from approx. 31 to 64. Within a single sporangium it was possible to detect formation of either only aborted spores or various mixtures of aborted and well-developed reduced spores and unreduced diplospores. The spores germinated in viable gametophytes with two ploidy levels: pentaploid (5x, from unreduced spores) and half of that (approx. 2·5x, from reduced spores). Moreover, 2-15% of gametophytes (both 2·5x and 5x) formed a viable sporophyte of the same ploidy level due to apogamy.

CONCLUSIONS

This study documents the mixed reproductive mode of a hybrid between apomictic and sexual ferns. Both sexual reduced and apomictic unreduced spores can be produced by a single individual, and even within a single sporangium. Both types of spores give rise to viable F2 generation gametophytes and sporophytes.

Reproductive and competitive interactions among gametophytes of the allotetraploid fern Dryopteris corleyi and its two diploid parents

BACKGROUND AND AIMS

Several models predict that the establishment of polyploids within diploid populations is enhanced by non-random mating (i.e. selfing and assortative mating) of cytotypes and by a higher relative fitness of polyploids. This report assesses the role that antheridiogens (i.e. maleness-inducing pheromones) and intercytotype differences in growth rate have on polyploid performance.

METHODS

Three buckler-fern species were studied: the allotetraploid Dryopteris corleyi and its diploid parents, D. aemula and D. oreades. In one experiment, gametophytes of these species were cultured under rich growth conditions to compare the timing of gametangia production. The substrata on which these gametophytes had grown were used as antheridiogen sources in a second experiment. The three species were combined as source and target of antheridiogen (i.e. nine species pairs). Timing of antheridia production and gametophyte size were determined after those antheridiogen treatments.

KEY RESULTS

Under rich growth conditions the allotetraploid produced archegonia earlier than those of diploid parents. Female gametophytes of the three species produced antheridiogens that inhibited growth and favoured maleness both within and among species. Gametophyte size was similar in the three species but antheridia formed earlier in the allotetraploid.

CONCLUSIONS

Unisexuality, promoted by non-specific antheridiogens, enhances random mating both within and among species. The resulting hybridization can favour the reproductive exclusion of the allopolyploid in sites where it is outnumbered by diploids. However, the earlier production of gametangia in the allotetraploid favours assortative mating and may thus counterbalance reproductive exclusion.

Overwintering leaves of a forest-floor fern, Dryopteris crassirhizoma (Dryopteridaceae): a small contribution to the resource storage and photosynthetic carbon gain

BACKGROUND AND AIMS

Dryopteris crassirhizoma is a semi-evergreen fern growing on the floor of deciduous forests. The present study aimed to clarify the photosynthetic and storage functions of overwintering leaves in this species.

METHODS

A 2-year experiment with defoliation and shading of overwintering leaves was conducted. Photosynthetic light response was measured in early spring (for overwintering leaves) and summer (for current-year leaves).

KEY RESULTS

No nitrogen limitation of growth was detected in plants subjected to defoliation. The number of leaves, their size, reproductive activity (production of sori) and total leaf mass were not affected by the treatment. The defoliation of overwintering leaves significantly reduced the bulk density of rhizomes and the root weight. The carbohydrates consumed by the rhizomes were assumed to be translocated for leaf production. Photosynthetic products of overwintering leaves were estimated to be small.

CONCLUSION

Overwintering leaves served very little as nutrient-storage and photosynthetic organs. They partly functioned as a carbon-storage organ but by contrast to previous studies, their physiological contribution to growth was found to be modest, probably because this species has a large rhizome system. The small contribution of overwintering leaves during the short-term period of this study may be explained by the significant storage ability of rhizomes in this long-living species. Other ecological functions of overwintering leaves, such as suppression of neighbouring plants in spring, are suggested.