plants Article Stability Despite Reduction: Flower Structure, Patterns of Receptacle Elongation and Organ Fusion in Eriocaulon (Eriocaulaceae: Poales) Dmitry D. Sokoloff 1 , Shrirang R. Yadav 2, Arun N. Chandore 3 and Margarita V. Remizowa 1,* 1 Biological Faculty, M.V. Lomonosov Moscow State University, 119234 Moscow, Russia; dmitry.sokoloff@msu-botany.ru or sokoloff[email protected] 2 Shivaji University, Vidyanagar, Kolhapur 416 004, MS, India; sryadavdu@rediffmail.com 3 Department of Botany, Abasaheb Marathe Arts and New Commerce, Science College, Rajapur 416 702, District Ratnagiri, MS, India; [email protected] * Correspondence: [email protected] or [email protected] Received: 6 October 2020; Accepted: 22 October 2020; Published: 24 October 2020 Abstract: Eriocaulaceae (Poales) differ from potentially related Xyridaceae in pattern of floral organ arrangement relative to subtending bract (with median sepal adaxial). Some Eriocaulaceae possess reduced and non-trimerous perianth, but developmental data are insufficient. We conducted a SEM investigation of flower development in three species of Eriocaulon to understand whether organ number and arrangement are stable in E. redactum, a species with a highly reduced calyx and reportedly missing corolla of female flowers. Early flower development is similar in all three species. Male and female flowers are indistinguishable at early stages. Despite earlier reports, both floral types uniformly possess three congenitally united sepals and three petals in E. redactum. Petals and inner stamens develop from common primordia. We assume that scanning electron microscopy should be used in taxonomic accounts of Eriocaulon to assess organ number and arrangement. Two types of corolla reduction are found in Eriocaulaceae: suppression and complete loss of petals. Common petal–stamen primordia in Eriocaulon do not co-occur with delayed receptacle expansion as in other monocots but are associated with retarded petal growth. The ‘reverse’ flower orientation of Eriocaulon is probably due to strictly transversal lateral sepals. Gynoecium development indicates similarities of Eriocaulaceae with restiids and graminids rather than with Xyridaceae. Keywords: common primordia; Eriocaulaeae; Eriocaulon redactum; floral development; floral evolution; flower; monocots; organ fusion; receptacle; Poales 1. Introduction The occurrence of trimerous pentacyclic flowers (with 3 + 3 perianth members, 3 + 3 stamens and 3 carpels) is a stable feature of many monocot lineages and one of key characteristics of monocots as a natural group [1–4]. Orientation of trimerous pentacyclic flowers in relation to surrounding phyllomes shows stable patterns across monocots [1,5]. In particular, trimerous flowers of almost all monocots lacking floral prophylls (bracteoles) on lateral pedicels have a median outer whorl perianth member inserted abaxially and two other outer whorl perianth members in transverse-adaxial positions (Figure1A). Exceptions from this rule are extremely rare and include the basal monocot family Araceae (Alismatales: [6]) and the commelinid family Eriocaulaceae (Poales: [1,7–11]), where the outer whorl median member of the trimerous perianth occupies an adaxial position (Figure1B). However, Araceae are also remarkable in the total absence of flower-subtending bract (that plays important morphogenetic roles in other monocots) and overall high variation of flower groundplan [6,12]. In contrast, flower-subtending bracts are well developed at least in the peripheral flowers in Plants 2020, 9, 1424; doi:10.3390/plants9111424 www.mdpi.com/journal/plants Plants 2020, 9, 1424x FOR PEER REVIEW 2 of 26 high variation of flower groundplan [6,12]. In contrast, flower-subtending bracts are well developed atEriocaulaceae least in the peripheral and trimerous flowers pentacyclic in Eriocaulacea flowerse and are trimerous common pentacyclic in one of theflowers two are largest common genera in oneof the of family, the twoEriocaulon largest[ 9genera,13]. Therefore, of the family, trimerous Eriocaulon flowers of[9,13]. Eriocaulaceae Therefore, are trimerous nearly unique flowers among of Eriocaulaceaemonocots with respectare nearly to their unique orientation among monocots pattern. with respect to their orientation pattern. Figure 1. Flower diagrams. (A) Typical trimerous pentacyclic monocot flower with subtending bract Figure 1. Flower diagrams. (A) Typical trimerous pentacyclic monocot flower with subtending bract and no floral prophylls. (B) Reverse orientation of trimerous pentacyclic flowers. (A,B) are generalized and no floral prophylls. (B) Reverse orientation of trimerous pentacyclic flowers. (A) and (B) are diagrams. (C) Flower diagram of Eriocaulon species studied here highlighting the fusion between sepals generalized diagrams. (C) Flower diagram of Eriocaulon species studied here highlighting the fusion and pronouncedly transversal position of lateral sepals. Relative arrangement of all organs is the same between sepals and pronouncedly transversal position of lateral sepals. Relative arrangement of all in flowers of both sexes. This diagram can be used as a legend to abbreviations adopted in all subsequent organs is the same in flowers of both sexes. This diagram can be used as a legend to abbreviations figures. Closed circles indicate inflorescence axis, black arcs represent the flower-subtending bract, adopted in all subsequent figures. Closed circles indicate inflorescence axis, black arcs represent the open arcs are perianth members, open ellipses are stamens, central triangle represents the gynoecium. flower-subtending bract, open arcs are perianth members, open ellipses are stamens, central triangle The gynoecium is sterile in functionally male flowers and the stamens are sterile (staminodia) in represents the gynoecium. The gynoecium is sterile in functionally male flowers and the stamens are functionally female flowers. fsb, flower-subtending bract; lc, lateral carpels; lis, lateral inner whorl sterile (staminodia) in functionally female flowers. fsb, flower-subtending bract; lc, lateral carpels; lis, stamens; los, lateral outer whorl stamens; lp, lateral petals; lse, lateral sepals; mc, median carpel; mis, lateral inner whorl stamens; los, lateral outer whorl stamens; lp, lateral petals; lse, lateral sepals; mc, median inner whorl stamen; mos, median outer whorl stamen; mp, median petal; mse, median sepal. median carpel; mis, median inner whorl stamen; mos, median outer whorl stamen; mp, median petal;Eriocaulaceae mse, median are sepal. well-known for their characteristic insect-pollinated compact racemose (partial) inflorescences. Surrounded by sterile involucral bracts, they resemble capitula of Asteraceae and some otherEriocaulaceae specialized eudicots are well-known [11,14–16]. Similarfor their compact characteristic inflorescences insect-pollinated are found in anothercompact family racemose of the (partial)order Poales, inflorescences. Xyridaceae, Surrounded which may by (or sterile may involucral not) be sister bracts, to Eriocaulaceae they resemble [17 capitula–19], but of orientationAsteraceae andof trimerous some other flowers specialized follows eudicots the typical [11,14–16]. monocot Si patternmilar compact in Xyridaceae inflorescences [11,20]. are Although found in being another tiny familyand uniformly of the order colored, Poales, the perianthXyridaceae, of Eriocaulaceae which may (or is believed may not) to be disisterfferentiated to Eriocaulaceae into a calyx [17–19], and a butcorolla. orientation This feature of trimerous is shared flowers by Eriocaulaceae, follows the Xyridaceae typical monocot and another pattern potentially in Xyridaceae related [11,20]. family, AlthoughMayacaceae being [19,21 tiny,22], butand in uniformly contrast with colored, the two the other perianth families, of members Eriocaulacea of Eriocaulaceae,e is believed with to very be differentiatedfew scattered reversals,into a calyx possess and functionallya corolla. This unisexual feature flowersis shared [13 by,23 ,Eriocaulaceae,24]. Xyridaceae and anotherThe potentially unusual nature related of family, orientation Mayacaceae of trimerous [19,21 pentacyclic,22], but in flowerscontrast in with Eriocaulaceae the two other highlights families, a membersneed of more of Eriocaulaceae, detailed comparative with very and few developmental scattered reversals, studies possess with attentionfunctionally to variation unisexual of flowers flower [13,23,24].groundplan in the family. The most obvious aspects of floral diversity in Eriocaulaceae are the reductionThe unusual of the outer nature whorl of orientation of the androecium of trimerous (its pentacyclic rudiments flowers can be oftenin Eriocaulaceae traced in male highlights flowers) a needin Paepalanthoideae, of more detailed occurrence comparative of dimeryand developmenta rather thanl trimery studies inwith some attention taxa, patterns to variation of corolla of flower tube groundplanformation and in petalthe family. to stamen The fusion most [obvious9,11,13,25 aspect–28].s In of the floral present diversity study, in we Eriocaulaceae explore the reported are the phenomenonreduction of the of outer the absence whorl of the the androecium inner perianth (its whorlrudiments (corolla) can be in femaleoften traced flowers in male of some flowers) species in Paepalanthoideae,of Eriocaulon, e.g., [occurrence9,29]. Complete of dimery loss of rather only onethan perianth trimery whorlin some is ataxa, rare patterns condition of incorolla monocots, tube formationexcept some