Cytological Study of Begonia Sections Flocciferae, Parvibegonia and Platycentrum (Begoniaceae) with Chromosome Numbers 2n=28 and 56

Abstract

Previous chromosome information for 2n=28 is restricted to 17 species in four Asian Begonia sections Baryandra, Diploclinium, Petermannia and Reichenheimia. Here we report the first chromosome counts and karyotype analyses of 2n=28 for three species in two Asian sections Flocciferae and Parvibegonia, as well as those for 2n=56 of B. baliensis in sect. Platycentrum. The number of sections with 2n=28 reported is increased to six of 20 sections of Asian Begonia. Based on the karyotypes, the chromosome evolution for 2n=28 as well as 2n=56 in Asian Begonia is discussed.

Begonia L., one of the largest genera of angiosperms, is a pantropical genus of 2,115 described species in 70 sections distributed in Africa, America, and Asia (Begonia Resource Centre: https://padme.rbge.org.uk/Begonia/home). Cytological studies revealed substantial variation in chromosome numbers in Begonia, including 2n=12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 31, 32, 33, 34, 36, 38, 40, 42, 43, 44, 45, 46, 48, 52, 54, 56, 60, 64, 66, 70, 71, 72, 73, 74, 76, 78, 82, 84 and 156 (Kono et al. 2020, 2022c). Based on chromosome observations of 100 species, Legro and Doorenbos (1969) concluded that the chromosome numbers of 2n=22 (14 spp.), 2n=28 (35 spp.) and 2n=56 (17 spp.) were the most common in Begonia. The conclusion made by Legro and Doorenbos (1969) is indeed evident in American Begonia of which the chromosome numbers 2n=28 and 2n=56 were reported in 87 species of 11 sections (Heitz 1927, Matsuura and Okuno 1936, Bowden 1945, Diers 1961, Legro and Doorenbos 1969, 1971, 1973, Doorenbos et al. 1998, Dewitte et al. 2009), or ca. 63% of the 137 species (in 20 sections) with cytological data (Heitz 1927, Matsuura and Okuno 1936, 1943, Hamel 1937, Bowden 1945, Diers 1961, Piton 1962, Favarger and Huynh 1965, Sharma 1970, Sharma and Bhattacharyya 1961, Legro and Doorenbos 1969, 1971, 1973, Sarkar 1974, 1989, Pinto-Magiio et al. 1997, Doorenbos et al. 1998, Dewitte et al. 2009). On the contrary, of the 18 African Begonia sections (Begonia Resource Centre: https://padme.rbge.org.uk/Begonia/home, Moonlight et al. 2018), 2n=28 and 2n=56 have only been reported in four species of sections Augustia (Klotzsch) A.DC. (Heitz 1927, Doorenbos et al. 1998) and Peltaugustia (Warb.) Barkley (Matsuura and Okuno 1936, 1943, Legro and Doorenbos 1969, Doorenbos et al. 1998, Marasek-Ciolakowska et al. 2010).

Amongst the 20 Asian sections (Begonia Resource Centre: https://padme.rbge.org.uk/Begonia/home, Moonlight et al. 2018, Krishna et al. 2020), 2n=28 have been reported in 17 species of sections Baryandra A.DC. (Kono et al. 2021a), Diploclinium (Lindl.) A.DC. (Legro and Doorenbos 1969), Petermannia (Klotzsch) A.DC. (Piton 1962) and Reichenheimia (Klotzsch) A.DC. (Li et al. 2005), and 2n=56 in one species of sect. Baryandra (Kokubugata and Madulid 2000) and one species of sect. Lauchea (Klotzsch) A.DC. (Legro and Doorenbos 1969, Doorenbos et al. 1998), respectively.

Because Begonia chromosomes are small and shorter than 2 µm long, karyotype analyses of 2n=28 have only been reported in three species of sect. Baryandra (Kono et al. 2021a). In this paper, we provide the first report of karyotypes of three species with 2n=28 in sections Flocciferae N.Krishna & Pradeep and Parvibegonia A.DC., and one species with 2n=56 in sect. Platycentrum (Klotzsch) A.DC. Based on the present results and previous chromosome reports for a total of 22 species with 2n=28 and 2n=56, the chromosome evolution for 2n=28 and 56 of Asian Begonia is discussed.

Materials and methods

The species of Asian Begonia with 2n=28 and/or 56 investigated are listed in Table 1 along with collection data. The sectional classification of Asian Begonia followed Moonlight et al. (2018) and Krishna et al. (2020). Somatic chromosomes were examined using root tips. Methods of pretreatment, fixation, maceration and staining for chromosome observations were described by Kono et al. (2020). Categories of chromosome morphology based on the position of centromeres followed by Levan et al. (1964).

Table 1. Chromosome data in Asian Begonia with 2n=28 and 56.

Section/species	Voucher information1)	Chromosome number and Karyotype formula2)	Reference
Sect. Baryandra A.DC.
B. camiguinensis Elmer		2n=28=10m+18sm(st)	Kono et al. (2021a), Nakamura et al. (2021)
B. chingipengii Rubite		2n=28	Rubite et al. (2014), Kono et al. (2021a)
B. droseroides C.I Peng, Rubite & C.W.Lin		2n=28	Kono et al. (2021a)
B. elmeri Merr.		2n=28=10m+18sm(st)	Kono et al. (2021a)
B. fenicis Merr.		2n=28	Nakamura et al. (2013)
		2n=56	Kokubugata and Madulid (2000)
B. gabaldonensis Rubite, C.I Peng & C.W.Lin		2n=28=10m+18sm(st)	Kono et al. (2021a)
B. hernandioides Merr.		2n=28	Kono et al. (2021a)
B. merrilliana C.I Peng, Rubite, C.W.Lin & K.F.Chung		2n=28	Peng et al. (2017), Kono et al. (2021a)
B. mindorensis Merr.		2n=28	Kono et al. (2021a)
B. neopurpurea L.B.Sm. & Wassh.		2n=28	Kono et al. (2021a)
B. sykakiengii Rubite, C.I Peng, C.W.Lin & K.F.Chung		2n=28	Peng et al. (2017), Kono et al. (2021a)
B. tandangii C.I Peng & Rubite		2n=28	Nakamura et al. (2013), Kono et al. (2021a)
B. tayabensis Merr.		2n=28	Kono et al. (2021a)
B. taraw M.Hughes, C.I Peng & Rubite		2n=28	Rubite et al. (2015), Kono et al. (2021a)
Sect. Diploclinium (Lindl.) A.DC.
B. acaulis Merr. & L.M.Perry		2n=28	Legro and Doorenbos (1969)
Sect. Flocciferae N.Krishna & Pradeep3)
B. albococcinea Hook.	Cult., Academia Sinica, Peng 23302	2n=28=12m+16sm(st)2sm	Present study
B. floccifera Bedd.		2n=28–32	Legro and Doorenbos (1971)
		2n=30	Sharma and Bhattacharyya (1957)
		2n=32	Doorenbos et al. (1998)
	India, Peng 21216	2n=28=12m+16sm(st)2sm	Present study
Sect. Lauchea (Klotzsch) A.DC.
B. crenata Dryand.		2n=56	Legro and Doorenbos (1969),
			Doorenbos et al. (1998)
Sect. Parvibegonia A.DC.
B. variabilis Ridl.	Thailand, Peng 21040	2n=28=12m+16sm(st)2sm	Present study
Sect. Petermannia (Klotzsch) A.DC.
B. isoptera Dryand. ex Sm.		2n=28	Piton (1962)
Sect. Platycentrum (Klotzsch) A.DC.
B. baliensis Girm.	Indonesia, Bali, Peng 21304	2n=56=20m+36sm(st)4sm	Present study
Sect. Reichenheimia (Klotzsch) A.DC.
B. parvula H.Lév. & Vaniot		2n=28	Li et al. (2005)

¹⁾All collections are kept at HAST. ²⁾m, chromosome with a centromere at median position; sm, chromosome with a centromere at submedian position; st, chromosome with a centromere at subterminal position; sm, longer sm(st)-chromosome with SC. ³⁾Sectional classification followed Krishna et al. (2020)

Results

Chromosome cytology of B. albococcinea Hook. in sect. Flocciferae, was studied for the first time. Chromosomes at mitotic metaphase are 2n=28 (Fig. 1a, b, g). The size of chromosomes varies gradually; the longest chromosome is ca. 2.0 µm long, and the shortest is ca. 1.1 µm long. Among 28 chromosomes, 12 chromosomes (Nos. 1–12 in Fig. 1g) have centromeres at the median position (m-chromosome) and the other 16 chromosomes (Nos. 13–28 in Fig. 1g) have centromeres at the submedian position (sm-chromosome) or subterminal position (st-chromosome). Among 28 chromosomes, ten sm(st)-chromosomes (Nos. 13–18, 23–24, 27–28 in Fig. 1g) have secondary constriction and/or small constriction (SC). Among 16 sm(st)-chromosomes, two with SC (sm-chromosome) (Nos. 27–28 in Fig. 1g) are longer than others. Satellites are observed at the distal region of the short arm of a pair of sm(st)-chromosomes (Nos. 23–24 in Fig. 1g). Karyotype formula is 2n=28=12m+16sm(st)^2sm.

Fig. 1. Somatic chromosomes at metaphase of three Begonia species with 2n=28. (a and b) B. albococcinea in sect. Flocciferae. (c and d) B. floccifera in sect. Flocciferae. (e and f) B. variabilis in sect. Parvibegonia. b, d and f are drawings of a, c and e, respectively. g, h and i are chromosome arrangements of respective b, d and f based on the karyotypic formulae. Scale bar in a–f=5 µm, and in g–i=2 µm.

Karyotype of B. floccifera Bedd. in sect. Flocciferae was analyzed for the first time. Chromosomes at mitotic metaphase are 2n=28 (Fig. 1c, d, h), differing from previous reports of 2n=28–32 (Legro and Doorenbos 1971), 2n=30 (Sharma and Bhattacharyya 1957), and 2n=32 (Doorenbos et al. 1998). The size of chromosomes varies gradually; the longest chromosome is ca. 1.7 µm long, and the shortest is ca. 1.0 µm long. Among 28 chromosomes, 12 chromosomes (Nos. 1–12 in Fig. 1h) have centromeres at the median position and the other 16 chromosomes (Nos. 13–28 in Fig. 1h) have centromeres at the submedian or subterminal position. Among 28 chromosomes, eight sm(st)-chromosomes (Nos. 13–18, 27–28 in Fig. 1h) have SC. Among 16 sm(st)-chromosomes, two with SC (sm-chromosome) (Nos. 27–28 in Fig. 1h) are longer than others. Satellite chromosomes are not observed. The karyotype formula is 2n=28=12m+16sm(st)^2sm.

Chromosome cytology of B. variabilis Ridl. in sect. Parvibegonia was studied for the first time. Chromosomes at mitotic metaphase are 2n=28 (Fig. 1e, f, i). The size of chromosomes varies gradually; the longest chromosome is ca. 1.5 µm long, and the shortest is ca. 1.0 µm long. Among 28 chromosomes, 12 chromosomes (Nos. 1–12 in Fig. 1i) have centromeres at the median position and the other 16 chromosomes (Nos. 13–28 in Fig. 1i) have centromeres at the submedian or subterminal position. Among 28 chromosomes, eight sm(st)-chromosomes (Nos. 13–18, 27–28 in Fig. 1i) have SC. Among 16 sm(st)-chromosomes, two with SC (sm-chromosome) (Nos. 27–28 in Fig. 1i) are longer than others. Satellite chromosomes are not observed. The karyotype formula is 2n=28=12m+16sm(st)^2sm.

Chromosome cytology of B. baliensis Girm. in sect. Platycentrum, was studied for the first time. Chromosomes at mitotic metaphase are 2n=56 (Fig. 2a, b, c). The size of chromosomes varies gradually; the longest chromosome is ca. 1.9 µm long, and the shortest is ca. 0.9 µm long. Among 56 chromosomes, 20 chromosomes (Nos. 1–20 in Fig. 2c) have centromeres at the median position and the other 36 chromosomes (Nos. 21–56 in Fig. 2c) have centromeres at the submedian or subterminal position. Among 56 chromosomes, 12 chromosomes (Nos. 21–28, 53–56 in Fig. 2c) have SC. Among 36 sm(st)-chromosomes, four with SC (sm-chromosome) (Nos. 53–56 in Fig. 2c) are longer than others. Satellite chromosomes are not observed. The karyotype formula is 2n=56=20m+36sm(st)^4sm.

Fig. 2. Somatic chromosomes at metaphase of B. baliensis with 2n=56. (a) micrograph. (b) drawing of (a). (c) chromosome arrangement of (b) based on the karyotypic formula. Scale bar in a–b=5 µm, and in c=2 µm.

Discussion

Of Asian Begonia species with 2n=28, the karyotype has only been investigated in three species of the sect. Baryandra, which is 2n=28=10m+18sm(st) (Kono et al. 2021a). In the present study, an additional karyotype 2n=28=12m+16sm(st)^2sm was determined for three species of sections Flocciferae and Parvibegonia. Compared with the rearranged karyotype 2n=28=10m+18sm(st)^2sm of B. elmeri Merr. in sect. Baryandra (Fig. 3), the newly reported karyotype 2n=28=12m+16sm(st)^2sm (Fig. 1g, h, i) is also characterized by the presence of two sm-chromosomes, suggesting that the two sm-chromosomes might be common in all species with 2n=28 in sections Baryandra, Flocciferae and Parvibegonia.

Fig. 3. Chromosome rearrangement of B. elmeri in sect. Baryandra with 2n=28=10m+18sm(st)^2sm. Scale bar=2 µm.

Kono et al. (2021b) surmised that the chromosomal diversity in Asian Begonia likely originated from chromosome fusion and polyploidization. Given this, we hypothesize that the karyotype 2n=28=12m+16sm(st)^2sm (Fig. 4c) of B. albococcinea and B. floccifera could have originated from a putative species with 2n=32=20m+12sm(st) (Fig. 4a). Accordingly, the karyotype 2n=28=12m+16sm(st)^2sm might have originated from chromosome fusion of eight m-chromosomes (respective four black colored and stripe chromosomes, Nos. 5–8 in Fig. 4a) of 2n=32=20m+12sm(st), resulting in two sm(st)-chromosomes with SC (stripe chromosome, No. 9 in Fig. 4c) and two sm-chromosomes (black colored chromosome, No. 14 in Fig. 4c) in the 2n=28 karyotype. Alternatively, the karyotype 2n=28=12m+16sm(st)^2sm (Fig. 4c) could have originated from 2n=30=16m+14sm(st)^2sm (e.g., B. dipetala Graham of sect. Haagea; Kono et al. 2022a) by chromosome fusion of the four m-chromosomes (stripe chromosomes, Nos. 5–6 in Fig. 4b), resulting in the two sm(st)-chromosomes with SC (stripe chromosome, No. 9 in Fig. 4c). Given that B. dipetala, B. floccifera and B. albococcinea were phylogenetically all members of the Early-divergent Asia Begonia (EDAB) in Moonlight et al. (2018) and geographically all distributed in western Ghats (Begonia Resource Centre: https://padme.rbge.org.uk/Begonia/home, Krishna et al. 2020), southeastern India, it seems more plausible that sect. Flocciferae [i.e., 2n=28=12m+16sm(st)^2sm] should have originated from sect. Haagea [2n=30=16m+14sm(st)^2sm].

Fig. 4. A hypothesis of the chromosome evolution for 2n=28 in sections Flocciferae, Parvibegonia and Baryandra. (a) Haploid karyotype of putative 2n=32=20m+12sm(st). (b) Haploid karyotype of putative 2n=30=16m+14sm(st)^2sm. (c) Haploid karyotype of sections Flocciferae and Parvibegonia with 2n=28=12m+16sm(st)^2sm. (d) Haploid karyotype of putative 2n=30=8m+22sm(st)^2sm. (e) Haploid karyotype of 2n=28=10m+18sm(st)^2sm in sect. Baryandra.

As B. variabilis of sect. Parvibegonia is also characterized by the karyotype 2n=28=12m+16sm(st)^2sm, it could have evolved from either 2n=32=20m+12sm(st) or 2n=30=16m+14sm(st)^2sm. However, as sect. Parvibegonia is phylogenetically placed at the basal clades of the Asian Clade C closer to EDAB such as B. dipetala of sect. Haagea, 2n=28=12m+16sm(st)^2sm of B. variabilis is also like derived from chromosome fusion from 2n=30=16m+14sm(st)^2sm. Nevertheless, as sect. Parvibegonia consists of 28 species (Begonia Resource Centre: https://padme.rbge.org.uk/Begonia/home, Moonlight et al. 2018) and the chromosome numbers have been thus far known only from two species B. tenuifolia Dryand. with 2n=22 (Legro and Doorenbos 1971) and B. variabillis with 2n=28 reported in this study, additional cytological studies of the section are needed for critical discussion on chromosome evolution of the section. On the contrary, given the distant phylogenetic relationships between sect. Haagea and sect. Baryandra, the karyotype 2n=28=10m+18sm(st)^2sm in sect. Baryandra (Kono et al. 2021a) might be a derived karyotype from 2n=30, resulting from chromosome fusion of four sm(st)-chromosomes of 2n=30=8m+22sm(st)^2sm (stripe chromosomes, Nos. 11–12 in Fig. 4d) to a pair of m-chromosomes in 2n=28=10m+18sm(st)^2sm (stripe chromosomes, No. 1 in Fig. 4e).

The chromosome number 2n=56 in B. baliensis is the first such count in sect. Platycentrum (Kono et al. 2022b, 2022c). Its karyotype 2n=56=20m+36sm(st)^4sm consists of four sets of 14 chromosomes of n=5m+9sm(st)^1sm, suggesting that the species is tetraploid of 2n=28=10m+18sm(st)^2sm. As the karyotype 2n=28=10m+18sm(st)^2sm was also reported in three species of sect. Baryandra (Kono et al. 2021a), these species might be the parental and/or ancestral of B. baliensis. However, B. baliensis is classified as sect. Platycentrum that are distantly related to sect. Baryandra (Moonlight et al. 2018), though B. baliensis has never been analyzed using molecular data. As 2n=28 and 56 is found in a wide range of sections, further karyotypic analyses of other species with 2n=28 such as B. socotrana Hook.f. (Matsuura and Okuno 1936, 1943, Legro and Doorenbos 1969, Doorenbos et al. 1998, Marasek-Ciolakowska et al. 2010) of the African sect. Peltagustia (though native to the Socotra Island, Yemen), and 2n=56 such as B. crenata Dryand. (Legro and Doorenbos 1969, Doorenbos et al. 1998) of the Asian section Lauchea distributed in India and Myanmar (Begonia Resource Centre: https://padme.rbge.org.uk/Begonia/home) will be needed.

Acknowledgements

The authors thank the staff of the Experimental Greenhouse, Biodiversity Research Center, Academia Sinica for maintaining the collection. CIP and KFC were supported by funding from Academia Sinica.

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