Mating systems have important effects on the genetic diversity structure within and among populations in many flowering plants due to gene flow via pollen transference and seed dispersion. These factors directly affect the distribution of alleles and contribute to shaping the patterns of genetic diversity within and among populations (Hamrick, 1982; Loveless and Hamrick, 1984; Hamrick and Godt, 1996). Many studies on the reproductive ecology of plants have focused on field studies of reproductive systems and pollination aspects, along with the estimation of genetic and mating system parameters (Barrett and Eckert, 1990; Cascante-Marín et al., 2006; Eckert et al., 2009; Hmeljevski et al., 2011; Paggi et al., 2015). These studies provide valuable information regarding the spatial variation of mating systems and gene flow patterns in wild species (Moeller et al., 2017).

The outcrossing rate can be estimated by examining naturally-pollinated progeny arrays of plants. This approach excludes self-fertilised progeny from the data and uses a maximum-likelihood model (Ritland and Jain, 1981). In addition, conservation and evolutionary researchers concerned with different microevolutionary processes and the impact of landscape change must measure real-time gene movement. Austerlitz and Smouse (2001) and Smouse et al. (2001), developed a two-generation approach (TwoGener) to study pollen pool genetic structure, by quantifying the heterogeneity among the male gamete pools sampled by maternal plants distributed across populations, and estimating the mean pollination distance and effective neighbourhood size.

Genetic diversity and gene flow analyses are important for understanding the genetic diversity distribution of natural plant populations, which can influence local adaptation and provide valuable information for conservation decision-making (Segelbacher et al., 2021), although genetic diversity has often been neglected by environmental policy agreements (Hoban et al., 2021). Furthermore, genetic diversity is highly relevant to population viability, which can be reflected in survival and fecundity parameters determined by habitat conditions and biotic and abiotic factors (Reed and Frankham, 2003; Caruso et al., 2005; Leimu et al., 2006). Several fertility aspects have been studied to help estimate population viability, including plant size, flower production, fruit and seed set, and pollen and seed viability (Johnson et al., 2004; Paggi et al., 2007, 2013; Palma-Silva et al., 2008; Sampaio et al., 2012; Amat et al., 2013). Plant fruit and seed production can be influenced by the availability of biotic (pollen and pollinators; Barrett and Eckert, 1990; Colas et al., 2001; Johnson et al., 2004) and abiotic resources (Caruso et al., 2005). For example, pollen limitation on seed production can be a result of reduced pollinator visits or a decrease in the quantity or quality of pollen deposited per visit (Colas et al., 2001; Huang and Guo, 2002; Ashman et al., 2004; Buide, 2004; Ohara et al., 2006; Ratto et al., 2018; Bennett et al., 2020). In addition, fertility patterns associated with mating system analysis can provide a better understanding of biotic effects on the reproductive success of natural plant populations, especially when considering species from fragmented habitats (Aizen et al., 2002; Murren, 2002; Aguilar et al., 2006; Ohara et al., 2006), such as the Atlantic Forest (AF) located along the eastern coast of Brazil.

The relationship between plant fitness and patterns of genetic diversity differs between self-incompatible (positively correlated) and self-compatible (independent) species (Barrett, 2014). This relationship also affects population size and is a critical factor in plant ecology, evolution, and conservation. Even though many plant populations are naturally isolated and small, recent anthropogenic fragmentation of habitats has contributed to population reduction and isolation of several plant species (Leimu et al., 2006). In general, self-compatible species with large populations may experience less pronounced negative effects from habitat fragmentation (Leimu et al., 2006). Several studies have measured inbreeding depression, genetic variation, population size, and fitness of plant species to aid in their conservation (Stockwell et al., 2003; Kramer and Havens, 2009; Angeloni et al., 2011; Jacquemyn et al., 2012; Oakley and Winn, 2012; Frankham et al., 2014; Ottewell et al., 2016; Rodger et al., 2021).

Some species, such as epiphytic bromeliads live in stable, naturally fragmented habitats and are well-adapted to drought and wind, although their colonising ability is likely hindered by landscape structure, with only a limited number of suitable sites available (Benzing, 1990, 2000; Winkler et al., 2007; Victoriano-Romero et al., 2017). Bromeliad species frequently show high genetic differentiation among naturally fragmented populations, which could be a result of local adaptation by the species or genetic drift due to historical colonisation events (Zanella et al., 2012; Cascante-Marín et al., 2014; Hmeljevski et al., 2014; Goetze et al., 2016; Gonçalves-Oliveira et al., 2017; Godoy et al., 2018; Ruas et al., 2020; Vicente-Silva et al., 2021).

The Bromeliaceae family has approximately 3,140 described species (Butcher and Gouda, 2017), being among the most species-rich non-woody plant families in the Neotropics. It presents one of the most remarkable adaptive radiations in the plant world (Givnish et al., 2011, 2014). Around 50 percent of the species are epiphytic; they have evolved floral displays with a great diversity of colors, shapes, and scents that are related to pollinator attraction, with nectar being the usual reward (Zanella et al., 2012). These pollinators are bats, birds, and insects, in self- and outcrossing species; there are also autogamous taxa which do not need pollinator agents (Kessler and Krömer, 2000; Krömer and Kessler, 2006; Krömer et al., 2008; Kamke et al., 2011; Schmid et al., 2011a,b; Marques et al., 2015; Aguilar-Rodríguez et al., 2016). The species are also essential resource for small, hovering vertebrate pollinators in the New World (Fleming and Muchhala, 2008) and food sources for hummingbirds in many Neotropical forest regions (Dziedzioch et al., 2003; Wolowski et al., 2013; Pansarin and de Pedro, 2016; Nunes et al., 2018; Gomes et al., 2019; Lenzi and Paggi, 2020).

In the present study, we investigated patterns of spatiotemporal variation in mating systems and fertility of three populations of Vriesea gigantea Gaudichaud from the center of the range of the species to the southern margin over 2 years. Vriesea gigantea is a self-compatible species which showed high fertility parameters in one population evaluated (Paggi et al., 2007; Palma-Silva et al., 2008). Paggi et al. (2015) reported a high selfing rate (s = 0.612), and Sampaio et al. (2012) observed moderate to low inbreeding depression (δ = 0.02–0.39) in populations from its southern range margin. In addition to the reported high fertility, pollen limitation was observed due to habitat fragmentation (Paggi et al., 2007), although low pollen viability in some localities could also contribute to this effect (Palma-Silva et al., 2008). Palma-Silva et al. (2009) observed a latitudinal trend of decreasing genetic diversity from north to south away from the equator, consistent with the prediction of declining within-population and/or increasing among-population genetic diversity toward range margins (Eckert et al., 2008). For V. gigantea, Palma-Silva et al. (2009) hypothesised that the observed results are in concordance with the direction of historical forest expansion after the Last Maximum Glacial and the lack of gene flow at the current northern and southern range edges, which is partially explained by restricted seed dispersal (Paggi et al., 2010).

The focus of the present study was to evaluate the potential of plants to evolve local mating system adaptations, such as selfing, in relation to environmental changes (Eckert et al., 2010), for instance, edge effects and habitat loss. Specifically, we used our combined fertility and mating system data across populations and years to address the following questions: (1) Is there spatiotemporal variation in fertility and mating system parameters, and if so, how is this variation distributed in space and time? (2) Is there any correlation between spatiotemporal variation in outcrossing rates and fertility parameters? (3) What do estimates of the pollen pool genetic structure and the effective number of pollen donors tell us about gene flow among populations in different years?

In this study, we observed spatiotemporal variation in fertility, mating systems, and gene flow of wild populations of V. gigantea from the centre to the southern range edge of the species. Unfortunately, we found no populations in geographically intermediate localities among the studied populations. No significant differences were observed when comparing temporal variations. Nevertheless, spatial variation was observed from the range centre of the species (north) toward the southern range margin (south), with an evident trend of increasing pollen pool genetic structure, decreasing outcrossing rate and no apparent changes in fertility parameters with latitude. The sampled population in the extreme southern edge of the distribution (Taim) exhibited the highest selfing rate and strongest pollen pool genetic structure. The results presented here are important for ongoing debate regarding the cost of and evolutionary significance of the adaptation to selfing in plant mating systems.

Across populations and years, mating systems and fertility analyses revealed that the studied populations had both high selfing rates and high fertility values. We observed a large production of flowers, fruits, and seeds, and high seed germination rates in all populations. Nevertheless, a more detailed examination of the results revealed that each population behaved differently in terms of the mating system and fertility. Although the Itapuã and Maquiné populations had the highest t_m values, they showed the lowest fruit production, which could indicate pollen limitation due to ecological factors (Knight et al., 2005). In this case, qualitative (pollen viability) and/or quantitative (pollinator limitation) components, which had been previously identified in the Itapuã population as factors that decrease reproductive fitness, could be responsible for the observed results (Paggi et al., 2007; Palma-Silva et al., 2008).

Both populations showed high germination rates consistent with the low levels of inbreeding depression previously observed [δ = 0.05 in Maquiné, and δ = 0.03 in Itapuã; Sampaio et al. (2012)]. This result may indicate a significant contribution of outcrossing, although some unavoidable geitonogamy is expected to occur in self-compatible species that have stable low-to-intermediate selfing rates (Schoen and Lloyd, 1992; Lloyd, 1992; Porcher and Lande, 2005; Paggi et al., 2007, 2015). In contrast, the Taim population showed the highest fruit set, probably due to autonomous selfing or geitonogamy (Paggi et al., 2015). The results also indicated low cross-pollen competition, since we observed a high proportion of selfing (s = 0.848). Together, these features should effectively provide reproductive assurance for the species (Lloyd, 1992; Lloyd and Schoen, 1992); however, the Taim population had the lowest germination rate, likely due to inbreeding depression (δ = 0.02; Sampaio et al., 2012). Although we used a limited number of SSR loci to estimate these parameters, Palma-Silva et al. (2009) reported very similar results of low genetic diversity levels for Taim population (H_O = 0.039 and F_IS = 0.408) using ten SSR loci.

Variations in outcrossing rates across time and space can be attributed to the historical and biological contexts of each plant species (Loveless and Hamrick, 1984; Schemeske and Lande, 1985; Barrett and Eckert, 1990; Knight et al., 2005; Moeller et al., 2017). Most of this variation occurs in self-compatible species in which environmental and demographic factors can play essential roles in the degree of outcrossing due to differences in pollen quality and pollen vectors (Barrett and Eckert, 1990; Barrett, 2014). Those reproductive outcomes can vary among populations of animal-pollinated plants owing to differences in the availability of effective pollinators and environmental effects on plant phenotypes and their functional consequences for pollinator behavior that governs pollen dispersal (Tian et al., 2022). Here, we observed spatial inter-population variation in the mating system of V. gigantea, with an increase in selfing rates from north to south, and different effects on fertility parameters that can occur due to ecological or genetic factors (Barrett, 2014). Previous studies have shown that this species is self-compatible (Paggi et al., 2007) with some level of autonomous selfing and geitonogamy, although it also presents a cryptic self-incompatibility system with delayed germination of self-pollen at the stigma (Paggi et al., 2015). Moreover, Paggi et al. (2021) showed that hummingbirds are effective pollinators in the Southern populations of V. gigantea, even though the species is classified as chiropterophilous; the authors also reported only one hummingbird species for Itapuã and Taim populations, which may be related to the high levels of selfing observed here for both populations (Paggi et al., 2021). Additionally, low-to-moderate levels of inbreeding depression have been observed in the same population (Sampaio et al., 2012). Together, these features corroborate our observations of spatio-interpopulation variation in mating systems and fertility for the studied populations.

Results from the progeny analysis confirmed that selfing occurs at a moderate to high frequency (0.555 < s < 0.893; excluding data from Taim–2007), apparently as a consequence of different modes of self-fertilisation that contribute to reproductive assurance in V. gigantea populations when outcrossing is not possible. Many functional, ecological, morphological, and physiological factors affect the occurrence of self-fertilisation (Lloyd and Schoen, 1992). In the case of V. gigantea, this is more likely due to pollen limitation (lack of pollinator activity or low pollen quality; Paggi et al., 2007, 2021; Palma-Silva et al., 2008); however, demographic factors, such as genetic drift and consequently low genetic diversity could affect populations at the southern range margin, which have the highest selfing rates, and where natural populations have become scarce (personal observation). In many models for the selection of self-fertilisation, pollen limitation has been reported to be responsible for stable mixed mating systems with high selfing rates (Lloyd, 1992; Porcher and Lande, 2005), as was observed with V. gigantea. Models of mating system evolution that combine functional and genetic components have demonstrated that intermediate selfing is optimal for achieving the greatest possible fertility, implying that partial selfing may be adaptive and evolutionarily stable (Johnston et al., 2009). However, our results showed a slight positive correlation between outcrossing rates and fruit set in V. gigantea (Figure 2), indicating that outcrossing rates still have a significant influence on the fertility of this species, in which we observed lower fitness (germination rate) when high selfing rates occurred (Taim population, Tables 3, 4).

Fragmented habitats have a profound effect on the gene flow, mating patterns, and fertility of natural plant populations (Keller and Waller, 2002; Lienert, 2004; Aguilar et al., 2008). Previous molecular studies have indicated that V. gigantea populations at the margin of the species distribution range exhibit increased genetic drift due to reduced gene flow (N_em = 0.688) among populations, and present smaller effective population sizes (Ne) when compared to central populations located in the continuous forest (Palma-Silva et al., 2009). Additionally, we observed spatial inter-population variation in the outcrossing rates of V. gigantea, from low to moderate (south 0.107 < t_m < 0.445 north; Table 4 and Figure 3), and high levels of pollen pool structure (north 0.319 < Φ_FT < 0.598 south; Table 5 and Figure 3). These results corroborate the low levels of gene flow among populations from the southern range margin reported by Palma-Silva et al. (2009).

The fertility parameters revealed that, even with low gene flow, low effective population size (Palma-Silva et al., 2009), moderate inbreeding depression (Sampaio et al., 2012), strong pollen pool structure, and low effective number of pollen donors, all populations had high fitness levels (with high levels of fruit set and viable seed production). These results suggest that the observed among-population patterns of fertility, mating systems, and gene flow in V. gigantea may be attributable to adaptation for selfing, likely due to edge effects (Baker, 1955; Sun and Ritland, 1998; Pannell et al., 2015) during range expansion (during post-glacial migration; Palma-Silva et al., 2009; Moeller et al., 2017). However, other adaptive or non-adaptive casual factors cannot be excluded, such as pollinator behaviour or local densities of flowering plants.

By examining the spatiotemporal patterns of fertility, mating system, and gene flow parameters for all populations and years, we showed that V. gigantea presented a combination of high fertility, high selfing rate, and low gene flow among populations with a geographic distance higher than the average distance of pollination, which also resulted in a strong pollen pool structure. The Atlantic Forest is experiencing rapid ongoing fragmentation, and bottlenecks and low gene flow are expected to contribute to increasing genetic differentiation among populations in addition to high selfing rates. Considering the observed spatiotemporal variation, V. gigantea populations from the range edge, such as, the Taim population, should be granted conservation priority, since most of V. gigantea individuals occurs outside the conservation unit, and are threatened by habitat loss due to agriculture and livestock actions. Conservation strategies that protect populations at the range edge of the species may contribute to the continued survival and maintenance of the evolutionary potential and local adaptation of this species.

The original contributions presented in this study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

GP and FB designed the study. GP, CP-S, CZ, MG, and MB collected the material, performed the experiments, and analyzed the data. GP, CL, and FB wrote and reviewed the manuscript. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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