Introduction

Musa acuminata Colla is native to tropical Asia. It constitutes one of the two progenitors of most seedless banana cultivars (Cavendish; AAA Group). Several subspecies have been described based on their morphology, geographic distribution, cytology, and molecular biology (Cheesman 1948; Simmonds 1956, 1962; Perrier et al. 2009; Rouard et al. 2018; Šimoníková et al. 2022; Martin et al. 2023). Based on morphology and geographic distribution, Perrier et al. (2009) suggested nine subspecies of M. acuminata: subsp. banksii, subsp. burmannica, subsp. burmannicoides, subsp. errans, subsp. malaccensis, subsp. microcarpa, subsp. siamea, subsp. truncata, and subsp. zebrina, as well as three varieties: var. chinensis, var. sumatrana, and var. tomentosa.

Due to the large size and herbaceous habit of bananas, specimens are often deficient and difficult to compare (Nasution 1991; Wong et al. 2001). Several attempts have been made to describe the variation in M. acuminata at the subspecific level. Thus, Simmonds (1956) classified the complex using a combination of vegetative, floral, fruit, and ovary characters. The key primarily relied on sheath waxiness, pseudostem pigmentation, and several male bract features, including color, imbrication, persistence, and the presence of yellowish tips. Additional diagnostic characters included the fertility of female flowers, fruit shape (subobtuse vs. acuminate), and the number of ovules per ovary. Among these, male-phase inflorescence characters—particularly bract morphology and coloration—played a central role in delimiting subspecies. Later, De Langhe and Devreux (1960) used bract color, the pedicel, apex, and cross section of the fruit, bunch direction, and floral and fruit density as diagnostic features and erected a new subspecies, Musa acuminata subsp. burmannicoides De Langhe & Devreux, distributed in India. Nasution (1991) used reproductive, inflorescence, vegetative, and biogeographic characters to discriminate among 15 varieties of M. acuminata in Indonesia, including acuminata, microcarpa (Becc.) Nasution, and malaccensis (Ridl.) Nasution. The key relied primarily on floral and seed traits, including the sex of basal flowers, seed morphology, fruit indumentum, tepal shape, and staminode length, supplemented by male-phase inflorescence and bract morphology, leaf pigmentation, and geographic distribution. Later, De Langhe et al. (2000) reported an unknown form, informally referred to as pseudo-malaccensis, as well as several other allegedly subspecific hybrids within the M. acuminata populations found in northern Thailand. The identities of M. acuminata subsp. malaccensis (Ridl.) N.W.Simmonds, M. acuminata subsp. microcarpa (Becc.) Simmonds, and M. acuminata subsp. truncata (Ridl.) Kiew were also considered dubious (Häkkinen and De Langhe 2001) until Wong et al. (2001) clarified that they were genetically distinct taxa. Wong et al. (2001) emphasized the significant spatial separation of the subspecies. Thus, subsp. malaccensis and subsp. microcarpa occur in the lowlands at 600–900 m, whereas subsp. truncata is usually found above 900 m in mountainous areas. The subspecies malaccensis and truncata are distributed in mainland Malaysia, whereas subsp. microcarpa is restricted to the island of Borneo. Subsequent molecular analyses confirmed the status of these three subspecies (Wong et al. 2001).

Thailand is located in the transitional zone between the Indochinese and Sundaic biogeographic regions (Tougard 2001). Cheesman (1948: 27) noted that “the range of variation of the (M. acuminata) species in Siam (Thailand) is of great importance and much needs further study.” According to Simmonds (1960), the three M. acuminata subspecies—subsp. siamea Simmonds, subsp. burmannica Simmonds, and subsp. malaccensis (Ridl.) Simmonds—are widely distributed from southern India to Indochina in the north and from southern China to Borneo in the east. They overlap in Thailand along the Thailand–Myanmar border, in southern Thailand, and in the Isthmus of Kra Archipelago. However, no thorough study of M. acuminata across the entire Kingdom of Thailand has been conducted since Simmonds’ surveys in the 1950s and those of De Langhe et al. (2000), both of which were biased toward the northern region. Subspecies burmannica, burmannicoides, and siamea constitute both a morphological and a genetic complex with a geographic distribution across northeastern India, Burma, southern China, and Thailand. Molecular and cytogenetic data indicate that these three subspecies should be merged into subsp. burmannica (Perrier et al. 2009; Martin et al. 2020, 2023). Based on genetic evidence, it seems that this complex is closely related to subsp. malaccensis (Perrier et al. 2011). Genome assemblies indicated hybridization between subsp. burmannica and subsp. malaccensis, which is supported by their distributional overlap (Perrier et al. 2011; Rouard et al. 2018).

In a race against time to collect and protect valuable germplasm, several hundred M. acuminata accessions were collected and investigated throughout Thailand. This work has increased insight into the taxonomy of Musa acuminata at the subspecific level. A novel identification key to the subspecies of the M. acuminata complex in Thailand is proposed here.