Fitness of an allopolyploid rupicolous fern compared with its diploid progenitors: from sporogenesis to sporophyte formation

Local adaptation, recombination, and the fate of neopolyploids

Polyploidy is widely recognized as an important speciation mechanism because it isolates tetraploids from their diploid progenitors. Polyploidy also provides new genetic material that may facilitate adaptive evolution. However, new mutations are more likely to arise after a neopolyploid has already successfully invaded a population. Thus, the role of adaptive forces in establishing a polyploid remains unclear. One solution to this apparent paradox may lie in the capacity of polyploids to suppress recombination among preexisting locally adapted alleles. The local adaptation mechanism requires that spatially heterogeneous selection acts on multiple loci and that gene flow introduces maladapted alleles to the population where the polyploid forms. The mechanism requires neither strong genetic drift nor any intrinsic benefit of genome doubling and can accommodate any mode of gene action. A unique prediction of the mechanism is that adaptive alleles should predate polyploidization, a pattern consistent with observations from a few well-studied polyploids. The mechanism is also consistent with the coexistence of both diploid and tetraploid cytotypes, fitness heterogeneity among independently derived polyploids, and the prevalence of outcrossing among older polyploids. The local adaptation mechanism also makes novel predictions about circumstances favoring polyploid invasions that can be tested using molecular genetic or comparative approaches.

Polyploidy and environmental stress response: a comparative study of fern gametophytes

Climate change is rapidly altering natural habitats and generating complex patterns of environmental stress. Ferns are major components of many forest understories and, given their independent gametophyte generation, may experience unique pressures in emerging temperature and drought regimes. Polyploidy is widespread in ferns and may provide a selective advantage in these rapidly changing environments. This work aimed to understand whether the gametophytes of allopolyploid ferns respond differently to climate-related physiological stress than their diploid parents. The experimental approach involved a multifactorial design with 27 treatment combinations including exposure to multiple levels of drought and temperature over three treatment durations, with recovery measured at multiple timepoints. We measured Chl fluorescence from over 2000 gametophytes to evaluate stress avoidance and tolerance in diploid and polyploid species. Polyploids generally showed a greater ability to avoid and/or tolerate a range of stress conditions compared with their diploid counterparts, suggesting that polyploidy may confer enhanced flexibility and resilience under climate stress. Overall, these results suggest that polyploidy may provide some resilience to climate change in mixed ploidy populations. However, all species remain susceptible to the impacts of extreme drought and heat stress.

Leaf structure and water relations of an allotetraploid Mediterranean fern and its diploid parents

Allopolyploidy is a common speciation mechanism in plants; however, its physiological and ecological consequences in niche partitioning have been scarcely studied. In this sense, leaf traits are good proxies to study the adaptive capacity of allopolyploids and diploid parents to their respective environmental conditions. In the present work, leaf water relations (assessed through pressure-volume curves) and structural and anatomical traits of the allotetraploid fern Oeosporangium tinaei and its diploid parents, Oeosporangium hispanicum and Oeosporangium pteridioides, were studied under controlled conditions in response to a water stress (WS) cycle. O. hispanicum showed the lowest osmotic potential at turgor loss point (πtlp ) and leaf capacitance, together with higher leaf mass per area (LMA), leaf thickness (LT), leaf density (LD), and leaf dry matter content (LDMC), whereas O. pteridioides presented the opposite set of traits (high πtlp and capacitance, and low LMA, LT, LD, and LDMC). O. tinaei showed an intermediate position for most of the studied traits. The responsiveness (osmotic and elastic adjustments) to WS was low, although most of the traits explained the segregation of the three species across a range of drought tolerance according to the rank: O. hispanicum > O. tinaei > O. pteridioides. These trait differences may underlie the niche segregation among coexisting populations of the three species in the Mediterranean basin.

Ecophysiological Differentiation among Two Resurrection Ferns and Their Allopolyploid Derivative

Theoretically, the coexistence of diploids and related polyploids is constrained by reproductive and competitive mechanisms. Although niche differentiation can explain the commonly observed co-occurrence of cytotypes, the underlying ecophysiological differentiation among cytotypes has hardly been studied. We compared the leaf functional traits of the allotetraploid resurrection fern Oeosporangium tinaei (HHPP) and its diploid parents, O. hispanicum (HH) and O. pteridioides (PP), coexisting in the same location. Our experimental results showed that all three species can recover physiological status after severe leaf dehydration, which confirms their 'resurrection' ability. However, compared with PP, HH had much higher investment per unit area of light-capturing surface, lower carbon assimilation rate per unit mass for the same midday water potential, higher non-enzymatic antioxidant capacity, higher carbon content, and lower contents of nitrogen, phosphorus, and other macronutrients. These traits allow HH to live in microhabitats with less availability of water and nutrients (rock crevices) and to have a greater capacity for resurrection. The higher assimilation capacity and lower antioxidant capacity of PP explain its more humid and nutrient-rich microhabitats (shallow soils). HHPP traits were mostly intermediate between those of HH and PP, and they allow the allotetraploid to occupy the free niche space left by the diploids.

Apomictic fern fathers: an experimental approach to the reproductive characteristics of sexual, apomict, and hybrid fern gametophytes

PREMISE

Apomixis and hybridization are two essential and complementary factors in the evolution of plants, including ferns. Hybridization combines characteristics from different species, while apomixis conserves features within a lineage. When combined, these two processes result in apo-sex hybrids. The conditions leading to the formation of these hybrids are poorly understood in ferns.

METHODS

We cultivated spores from 66 fern samples (43 apomicts, 7 apo-sex hybrids, and 16 sexuals), and measured their development in vitro over 16 weeks. We evaluated germination, lateral meristem formation rates, sexual expression, and production of sporophytes and then compared ontogenetic patterns among the three groups.

RESULTS

The three examined groups formed antheridia (male gametangia) but differed in overall gametophyte development. Sexual species created archegonia (female, 86% of viable samples), but no sporophytes. Apomicts rarely created nonfunctional archegonia (8%) but usually produced apogamous sporophytes (75%). Surprisingly, apomictic and sexual species showed similar development speed. The sexually reproducing parents of viable studied hybrids formed about twice as many meristic gametophytes as the apomictic parents (39% vs. 20%, respectively).

CONCLUSIONS

We present the most thorough comparison of gametangial development of sexual and apomictic ferns, to date. Despite expectations, apomictic reproduction might not lead to earlier sporophyte formation. Apomicts produce functional sperm and thus can contribute this type of gamete to their hybrids. The development patterns found in the parents of hybrids indicate a possible increase of hybridization rates by antheridiogens. The apo-sex hybrids always inherit the apomictic reproductive strategy and are thus capable of self-perpetuation.

An experimental assessment of competitive interactions between sexual and apomictic fern gametophytes using Easy Leaf Area

PREMISE

Few studies have explored competition in fern gametophyte populations. One limiting factor is the tedious measurement of gametophyte size as a proxy for biomass in these small plants. Here, an alternative approach of estimating the number of green pixels from photos was employed to measure the competitive interactions among apomictic and sexual Dryopteris gametophytes.

METHODS

We cultivated the gametophytes of two apomictic (diploid and triploid) and one sexual (tetraploid) Dryopteris species in monocultures and in two-species mixtures in the ratios 1 : 1 and 1 : 3. The total gametophyte cover of each population originating from 20 spores was assessed using Easy Leaf Area. Assessments were performed weekly between weeks 4 and 10 of cultivation. Additionally, during week 5, the cover of each species in each mixture was estimated separately.

RESULTS

We identified a positive correlation between gametophyte size and ploidy level as well as sexual reproduction. The performance of the tested species in mixtures was dependent on the competitor species identity, indicating the importance of competition between gametophytes.

DISCUSSION

The methods outlined can be used for a rapid assessment of fern gametophyte cover in large gametophyte populations. Ploidy level and reproduction type seem to play a major role in the competitive abilities of fern gametophytes, but more research is needed on this topic.