Floral anatomy of Elatinaceae in a Malpighiales context
Compared to Malpighiaceae, the floral anatomy of Elatinaceae exhibits the following character reversions: absence of fused lateral sepal traces, lack of calyx and corolla adnation, and absence of a hypanthium (Table 1). However, a connate calyx, axile placentation, and pendulous ovules are present in Elatinaceae, Caryocaraceae, Lophopyxidaceae, Malpighiaceae, and Putranjivaceae.
Table 1
– Main floral anatomy characters observed in Elatinaceae, compared to those of the phylogenetically close families in Malpighiales (sensu Xi et al. 2012): Malpighiaceae, Centroplacaceae, Caryocaraceae, Putranjivaceae and Lophopyxidaceae. (?) indicates unknown data; (-) indicates lack of literature; (*) our personal observation; it indicates that characters described in the literature were referred applying the terminology adopted in this work.
Flower region | Characters | Elatinaceae | Malpighiaceae | Centroplacaceae | Caryocaraceae | Putranjivaceae | Lophopyxidaceae |
Pedicel | 1. Vascular cylinder organization | Siphonostele1 | Siphonostele or eustele24 | ? | Siphonostele or eustele11,14 | Siphonostele or eustele15 | Eustele15 |
Calyx | 2. Gamosepalous at some extesion | Present1,17 | Present3,5 | ? | Present11,14 | Present15 | Present (pistillate flowers) 15 |
3. Fusion of the lateral sepal traces | Not applicable1 | Present9,24 | ? | Present11,14 | Present15 | Absent15 |
4. Traces per sepal | 11 | 39,16,24 | ? | 3-many11,14 | 1 or 315 | 315 |
5. Origin of vasculature | Sepal traces1 | Sepal traces and sepal-petal complexes9,16,24 | ? | *Sepal traces and sepal-petal complexes11,14 | Sepal traces15 | Sepal traces and sepal-petal complexes? 15 |
6. Calyx glands on abaxial face of sepals | Absent1 | Present3,4 | ? | Present14 | Absent15 | Absent15 |
7. Type of gland | Vestigial1 | Elaiophores or nectaries or mixed secretion6,12,18,27,28 | ? | ? | Not applicable15 | Not applicable15 |
8. Fusion with the corolla | Absent1 | Present (ground and vascular tissues) 9,16,24 | ? | Present (vascular complexes)11,14 | Not applicable15 | Present (ground and vascular tissues) 15 |
9. Vasculature in the external region of the receptacle produced by basipetal sepal traces | Absent1 | Present9,16,24 | ? | Present11,14 | Absent (?)15 | Absent (?)15 |
Corolla | 10. Connate petals | Absent1,17 | Absent3,5 | ? | Absent11,14 | Not applicable15 | Absent15 |
11. Traces per petal | 11 | 1 (3) 9,16,24 | ? | 3-many11,14 | Not applicable15 | 115 |
12. Adnation to androecium | Present (rare)1 | Present (vascular complexes)9 | ? | Present11,14 | Not applicable15 | Absent15 |
Androecium | 13. Fusion degree among stamens | Free1 | Free or connate3,5 | ? | Connate11,14 | Free or connate15 | Free15 |
14. Traces per stamen | 11 | 19,16,24 | ? | 111,14 | 115 | 115 |
15. Anther glandular epidermis | Present (restricted to land species)1 | Present3,7,8,18 | ? | ? | ?15 | ?15 |
16. Occurrence of staminodes | Absent1 | Present3,5 | Present in pistillate flowers13 | Present11,14 | Absent15 | Absent15 |
Hypanthium | 17. Occurrence of the hypanthium (floral tube) | Absent1 | Present9 | ? | Present?11,14 | Present15 | Present15 |
18. If hypanthium is present, whorls involved | Not applicable1 | Perianth + androecium9 | ? | Corolla + androecium + gynoecium11,14 | Calyx + androecium or calyx + gynophore15 | Perianth (pistillate flowers) or perianth + androecium (staminate flowers)15 |
Gynoecium | 19. Number of carpels | 3–5 (rarely 2)1,10,20 | 33,5 | ? | 3-many11,14 | 1-many15 | 4-515 |
20. Styles fusion | Styles are free or fused1 | Styles are free or fused3,5 | ? | Free styles11,14 | Fused to the mid or apical region of the style15 | Fused to the mid or apical region of the style15 |
21. Differentiated carpel dorsal bundle | Present1 | Absent16,24,25, 26 | ? | Absent11,14 | Present15 | Present15 |
22. Ventral bundle per carpel | 11 | 19,16,24 | ? | 211,14 | 1-215 | 2?15 |
23. Fusion of ventral bundles between adjacent carpels | Present1 | Absent9,16,24 | ? | Absent11,14 | Absent?15 | Absent?15 |
24. Styles vascularized by adjacent carpels | Absent1 | Absent9,16,24 | ? | Present11,14 | Absent?15 | Absent?15 |
25. Occurrence of compitum | Present1 | Present2 | | Present14 | Present15 | Present15 |
26. Insertion of ovules | Basal or pendulum1 | Pendulum21,25 | ? | Pendulum 14 | Pendulum 15 | Pendulum 15 |
27. Type of placentation | Axile1,17,19 | Axile26 | ? | Axile11,14 | Axile15 | Axile15 |
28. Placenta invading locules | Present1 | Absent26 | ? | Absent11,14 | Present15 | Absent?15 |
29. Occurrence of hypostase | Present1 | Present21,22,25 | ? | Absent11,14 | Present15 | Absent?15 |
Secretory structures | 30. Occurrence of nectaries | Absent1 | Absent (in neotropical species)4 Present (in paleotropical species)12 | Present13 | Present11,14 | Present (Absent in Putranjiva)15 | Present15 |
31. If nectaries are present, position | Not applicable1 | Abaxial face of sepals12 | Receptacular?13 | Staminodes11,14 | Around the gynoecium15 | Inner surface of the hypanthium15 |
32. If nectaries are present, origin of vasculature | Not applicable1 | Calyx?12 | ? | Androecium11,14 | Gynoecium15 | Corolla15 |
Data sources: (1) This study; (2) Aliscioni et al. 2019; (3) Anderson 1979; (4) Anderson 1990; (5) Anderson et al. 2006; (6) Araújo and Meira 2016; (7) Arévalo-Rodrigues et al. 2020; (8) Avalos et al. 2020; (9) Bonifácio et al. 2021; (10) Dathan and Singh 1971; (11) Dickison 1990; (12) Guesdon et al. 2019; (13) Kubitzki 2014; (14) Matthews and Endress 2011; (15) Matthews and Endress 2013; (16) Mello 2017; (17) Niedenzu 1925; (18) Possobom et al. 2015; (19) Ramayya and Rajagopal 1971; (20) Salisbury 1967; (21) Souto and Oliveira 2005; (22) Souto and Oliveira 2008; (23) Souto 2011; (24) Souto and Oliveira 2013; (25) Souto and Oliveira 2014; (26) Souto and Oliveira 2020; (27) Vogel 1974; (28) Vogel 1990. |
A basally connate calyx, here observed only in Elatine, has also been reported for Caryocaraceae (Matthews and Endress 2011), Lophopyxidaceae (Matthews and Endress 2013), and Malpighiaceae (Anderson 1979). In these families, the calyx and corolla share a vascular complex (Dickison 1990; Matthews and Endress 2011, 2013; Souto and Oliveira 2013; Bonifácio et al. 2021), which represents the adnation of these whorls. This characteristic was lost in Elatinaceae and Putranjivaceae (Matthews and Endress 2013), and the latter lacks petals. Similarly, a hypanthium was lost in Caryocaraceae (Dickison 1990; Matthews and Endress 2011) and Elatinaceae (our data).
In the clades phylogenetically closest to the malpighioid clade (Xi et al. 2012), the presence of glands on the sepals of Malpighiaceae (Anderson 1979, 1990; Vogel 1990) and Caryocaraceae (Matthews and Endress 2011) is notable. In Malpighiaceae, the calyx glands are commonly elaiophores, or nectaries in some paleotropical lineages, which are between adjacent sepals (Anderson 1979, 1990; Vogel 1990; Souto and Oliveira 2013; Mello 2017; Bonifácio et al. 2021). We found that the sepal glands in Elatinaceae are vestigial and located only at the apex, with no distinction between the adaxial and abaxial surfaces. At the same time, the literature reports the presence of these glands only on the abaxial surface of the sepals (Endress et al. 2013). In Caryocaraceae, these glands are on the abaxial surface near the apex of the sepals. They are structurally similar to the elaiophores in Malpighiaceae, but their secretory nature is not known (Matthews and Endress 2011). The presence of floral nectaries, independent of the whorl they are on, occurs in all the close families in Malpighiales except in Elatinaceae (in this work): Caryocaraceae (Dickison 1990; Matthews and Endress 2011), Centroplacaceae (Kubitzki 2014b), Malpighiaceae (Guesdon et al. 2019), Lophopyxidaceae, and Putranjivaceae (Matthews and Endress 2013). The absence of nectaries is evidence that Elatinaceae diverged in the direction of autogamy and was independent of biotic pollination agents.
Evolutionary analyses of anatomical flower characters of Malpighiales revealed the relevance of the gynoecium (Endress et al. 2013), notably the presence of pendulous ovules, a characteristic of the COM clade (Celastrales, Oxalidales, and Malpighiales) (Endress 2011; Endress et al. 2013), and axile placentation, a well-established character in the malpighioide clade (Endress et al. 2013).
The fused ventral bundles observed in Elatinaceae have also been reported for Malpighiaceae (Souto and Oliveira 2013, 2020; Mello 2017). However, unlike Malpighiaceae, in Elatinaceae, intercarpellary complexes vascularize two carpels simultaneously (as recorded here in Bergia perennis and Elatine lindbergii), indicating lateral fusion between carpels. The presence of intercarpellary complexes has been recorded in other families of Malpighiales, such as Euphorbiaceae (our observation in Fig. 4i of De-Paula et al. 2011), Linaceae (our observation in Fig. 24l of Matthews and Endress 2011), Putranjivaceae (our observation in Fig. 13e of Matthews and Endress 2013), Rhizophoraceae (our observation in Fig. 5n of Matthews and Endress 2011), and Dichapetalaceae and Trigoniaceae (our observation in Figs. 13p and 17j, respectively, of Matthews and Endress 2008). However, sharing ventral bundles between adjacent carpels is not commonly reported; it has been recorded in Anthodiscus (Caryocaraceae), where each style receives one bundle from adjacent carpels (Dickison 1990). The presence of intercarpellary complexes could be an interesting character in investigating the lateral fusion and the numeric variation of carpels, which are very recurrent in Malpighiales (Endress et al. 2013).
Potential Synapomorphies Between Elatinaceae And Malpighiaceae
The flower characters shared between Elatinaceae and Malpighiaceae observed in this study are the presence of a glandular connective and hypostase. The presence of a glandular connective has never been reported for Elatinaceae. It is variable among species since it was not observed in the plants with an aquatic habit (Elatine gratioloides and E. triandra). The glandular connective is possibly related to a terrestrial habit because it is present in species that occupy this type of environment (Bergia perennis [Leach 1989] and the amphibious E. lindbergii [Bittrich 2002]). Considering that the aquatic lineage of Elatinaceae diverged more recently, in contrast to the terrestrial lineages with secondary growth (Davis and Chase 2004), the presence of a glandular connective is plesiomorphic for the family.
For Malpighiaceae, the presence of secretory structures on the anther connective has been known for some time (Gates 1982; Anderson 1990) and repeatedly reported in the literature (Possobom et al. 2015; Arévalo-Rodrigues et al. 2020; Avalos et al. 2020). These glands are composed of globose epidermal cells with phenolic content in the vacuole and secrete oil (Arévalo-Rodrigues et al. 2020) or are glands that secrete a mixture of oil and sugars (Possobom et al. 2015).
In relation to the ovules, the presence of a hypostase has also been reported for Malpighiaceae in Byrsonima (Souto and Oliveira 2005), Banisteriopsis and Diplopterys (Souto and Oliveira 2008), and Janusia, Mascagnia, and Tetrapterys (Souto and Oliveira 2014). Further, a hypostase has been reported for other families of Malpighiales, such as Euphorbiaceae s.l. (Landes 1946; Carmichael and Selbo 1999) and Chrysobalanaceae (Tobe and Raven 1984). Since ovule characters are significant in superfamilial analyses in Malpighiales (Endress et al. 2013), future studies comparing Malpighiaceae and Elatinaceae and including more clades could provide more information about the importance and constancy of these characters. In the same way, characters associated with seeds are conserved in Elatine, unlike its vegetative organs that exhibit high phenotypic plasticity when submitted to environmental variations (Molnár et al. 2015).
The gynoecium characters that differ the most between Elatinaceae and Malpighiaceae, such as more carpels and numerous ovules in Elatinaceae, are related to greater investment in the female flowers associated with attracting pollinators in Malpighiaceae. The evolution of Malpighiaceae was especially influenced by a mutualistic relationship with oil-collecting bees, resulting in allogamy and dependence on pollinators (Sigrist and Sazima 2004). In contrast, Elatinaceae tend to not depend on pollinators and instead rely on autogamy (Uphof 1938; Tucker 1986; Razifard et al. 2017a).
Some data about the biogeography of Elatinaceae point to the origin and distribution of the family in the direction of temperate regions (Cai et al. 2016). This information seems to go against some of the advantages commonly related to autogamy: colonization from a single propagule, less investment in the androecium, and successfully reproducing in locations where pollinators are limited, for example, in artic environments (Baker 1955; Cheptou 2012; Pannell 2015).
The two families are currently circumscribed in the clade [Elatinaceae + Malpighiaceae] + Centroplacaceae, characterized by axile placentation, crassinucellate ovules, no endothelium, and persistent sepals on the fruits (Xi et al. 2012). Support for the clade is low due to the absence of morphological information for Centroplacaceae, and floral anatomy and embryological data of this family could help better explain the evolution of the clade.
The Floral Anatomy Of Elatinaceae
The floral anatomy analysis of Elatinaceae revealed that the presence of a siphonostele is a characteristic exclusive to this group among the closest Malpighiales (Table 1). The remaining characteristics, in contrast to the other Malpighiales families, reinforce how selection acted on the floral architecture of Elatinaceae in favor of autogamy and partially explain the numerous morphological differences between this family and Malpighiaceae.
The anatomical characters that are evidence of autogamy are the reduction in the number of microsporangia (Endress and Stumpf 1990; Battjes et al. 1994), precocious germination of pollen grains inside the anthers (Mazer and Delesalle 1998; Mann et al. 2021), and the small investment in attracting and rewarding pollinators, including the absence of floral nectaries (Mazer and Delesalle 1998; Goodwillie et al. 2010).
The absence of physical barriers that prevent self-fertilization is a strong indicator of floral adaptations to autogamy (Garnock-Jones 1976). In this work, the most extreme modifications to self-fertilization observed were the gynostegium and germination in situ. Pollen grain germination in situ can occur in both chasmogamous and cleistogamous species but must occur in cleistogamous species (Lord 1981). Interestingly, there are records of this type of process in Malpighiaceae; pollen grains have been observed germinating inside indehiscent anthers of cleistogamous flowers of Gaudichaudia spp. and Janusia guaranitica A. Juss. (Anderson 1980).
From an ecological perspective, autonomous self-fertilization is a strategy that guarantees reproduction (Zhang and Li 2008) and promotes establishment after long-distance dispersal when crossbreeding is not possible (Baker 1955; Llyoid 1992; Morgan and Wilson 2005; Passos et al. 2021). This can be related to the emergence of Elatinaceae, which underwent long-distance dispersal during the Eocene long after the emergence of Malpighiaceae at the beginning of the Cretaceous (Cai et al. 2016).
Since there are herbaceous species with an aquatic habit in Elatinaceae (Davis and Chase 2004), many anatomical floral characteristics of this group result from environmental adaptations and often evolutionary reversals. However, it is still possible to infer the evolution of the floral structure in the group based on its floral anatomy.
Of the characters observed, the presence of vascular complexes feeding adjacent carpels, placenta projecting to the interior of the locule, and hypostase on the ovules occur in all the Elatinaceae species we studied. The presence of differing adaxial and abaxial sepal surfaces was exclusively observed in Bergia, while gamosepalous calyx are only present in Elatine; these are possible synapomorphies for the genera.
Although uncommon in angiosperms, a reduction to one sepal trace is understandable considering physiological aspects (Puri 1951) since the plants are reduced in size, with tiny flowers, and adapted to aquatic environments. The reduced flowers can also be explained by a decrease in allocating resources to floral display (Goodwillie et al. 2010); autogamy is known to occur in some family species and, consequently, these species do not require pollinators (Uphof 1938; Tucker 1986).
Autogamy in Elatinaceae is frequently associated with cleistogamy in submersed flowers (Uphof 1938; Tucker 1986). However, we do not know if all the species we analyzed are cleistogamous because there is only information about the reproductive system of Elatine triandra (Uphof 1938). Despite this, aquatic, cleistogamous plants commonly lack endothecium thickening, as recognized here for E. triandra, since the wall of the microsporangia does not differentiate for active dehiscence (Maheshwari 1950). Additionally, in cleistogamous plants, a reduced corolla and reduced androecium are expected (Lord 1981). Other evidence of cleistogamy observed in this work was the androecium fused to the gynoecium in Elatine gratioloides, which points to an extreme specialization that favors autogamy. This fusion between the anthers of antisepalous stamens and the stigmas forms the gynostegium (sensu Endress 2016).