Results

The evidence obtained from 211 accessions collected over 20 years revealed a remarkable diversity of M. acuminata throughout Thailand and allowed the geographic and altitudinal distributions of its subspecies to be mapped (Figs 1, 2). Only a few wild Musa species were recorded in northeastern and eastern Thailand, most probably because of forest clearing and human disturbance.

Figure 2.: A two-dimensional plot of the 199 Musa acuminata accessions collected in Thailand as part of this study. The x-axis represents latitude, and the y-axis represents altitude. The four subspecies of M. acuminata are color-coded. The green dashed line and arrow indicate the transition between subsp. siamea and subsp. kraburiensis near the Isthmus of Kra at 10°11'N, latitude (black dashed line and arrow).

Musa acuminata subsp. siamea is the most widely distributed subspecies. It has predominantly been recorded on the central plain, with extensions northward, southeastward, and southwestward. Musa acuminata subsp. kraburiensis, a newly described subspecies recorded from sea level to 300 m altitude, has a restricted range between 9°N and 11°N. This corresponds to the transition between the Tanao Si/Tenasserim and Phuket mountain ranges on the Isthmus of Kra in the provinces of Chumphon and Ranong. Subspecies malaccensis is distributed throughout Peninsular Thailand. It was also recorded in northern Thailand at elevations of up to 1,000 m, which is considerably higher than the previously reported 300 m. The fourth subspecies, M. acuminata subsp. truncata, is newly recorded for Thailand. It was observed along the Thai border with Malaysia in the Sankalakhiri and Titiwangsa mountain ranges in the provinces of Narathiwat and Yala. In this area, it occurs sympatrically with subsp. malaccensis at lower elevations, and several putative hybrids between these two subspecies were observed.

Discriminating morphological characters

A multivariate analysis of 16 morphological characters (Table 1, Suppl. material 2) revealed several differences of diagnostic value among the four Thai subspecies of M. acuminata (Fig. 6). These included vegetative features such as the shape of the leaf base (Fig. 3), rachis position (Fig. 4), and shape of the male-phase inflorescence, as well as the arrangement and color of the bracts (Fig. 5). The cluster analysis revealed two major morphological clusters determined mainly by the position of the inflorescence. The first cluster (cluster I), which corresponds to the new subsp. kraburiensis, exhibited a horizontal to upward-curving inflorescence/infructescence rachis. The second cluster (cluster II), which comprises the accessions of the remaining three subspecies, is characterized by a downward-pointing inflorescence/infructescence rachis, ranging from curved to drooping. Within cluster II, the shape of the leaf base, bract arrangement, and color were all of diagnostic value for distinguishing among subspecies. Musa acuminata subsp. siamea or subsp. burmannica sensu Simmonds is characterized by cuneate to oblique leaf bases and a conspicuously imbricate arrangement of the rachis bracts. In contrast, M. acuminata subsp. malaccensis and subsp. truncata stood out by having round, cordate, or auriculate leaf bases and slightly imbricate to non-imbricate rachis bracts. Furthermore, the rachis bracts of all three subspecies constituting cluster II exhibited distinct coloration (Fig. 5). Within cluster II, subsp. truncata differs from the sympatric subsp. malaccensis in its unique bract coloration. In subsp. truncata, the inflorescence bracts are dark purple externally (abaxially) and purplish red or ivory internally (adaxially), sometimes with a pale red tinge. By contrast, the bracts of subsp. malaccensis are red, purplish red, or pinkish purple on both surfaces (Fig. 5). The UPGMA analysis of morphological characters separated the formae of subsp. kraburiensis and subsp. siamea. Within subsp. kraburiensis, f. luteola, characterized by its yellow bracts, formed a distinct cluster separate from f. kraburiensis. Although subsp. siamea f. nanakornii possesses numerous male buds, a feature uncommon among other taxa of M. acuminata, its cuneate to oblique leaf base supports placement within subsp. siamea.

Figure 3.: Variation in the leaf bases observed in the four subspecies of Musa acuminata recorded in Thailand. A. Cuneate—subsp. siamea, Swangpol & Somana 091, 182, 262; B. Oblique—subsp. siamea, Swangpol & Somana 021, 247, 266; C. Round—subsp. kraburiensis, Swangpol & Somana 107, 108, 142; subsp. malaccensis, Swangpol & Somana 204, 274; subsp. truncata, Swangpol & Somana 206, 490, 497; D. Auriculate—subsp. kraburiensis, Swangpol & Somana 009; subsp. malaccensis, Swangpol & Somana 005, 006, 010; E. Cordate—subsp. malaccensis, Swangpol & Somana 082, 138, 546. Illustrations by Potjana Kaittiprapai.

Figure 4.: Variation in the position of the inflorescence/infructescence rachis observed in the four subspecies of Musa acuminata recorded in Thailand. A, B. Drooping at an angle—subsp. siamea, Swangpol & Somana 172, 007; C. Curved downward—subsp. siamea, Swangpol & Somana 247; D. Downward at an angle—subsp. truncata, Swangpol & Somana 206; E, F. Horizontal with slight (E) or pronounced curvature (F)—subsp. malaccensis (typical form), Swangpol & Somana 204, 203; (G) horizontal to pointing upward—subsp. kraburiensis, Swangpol & Somana 104; H. Curved upward—subsp. kraburiensis, Swangpol & Somana 107. Illustrations by Potjana Kaittiprapai.

Figure 5.: Variation in the overall shape, arrangement, and color of the apical male flower cluster observed across the four Musa acuminata subspecies recorded in Thailand: A–C. Subsp. kraburiensis—Swangpol & Somana 108, 107, 142; D–F. Subsp. malaccensis—Swangpol & Somana 140, 485, 138; G–I. Subsp. siamea—Swangpol & Somana 021, 091, 266; J, K. Subsp. truncata—Swangpol & Somana 491, 497; L. Comparison of subsp. truncata—Swangpol & Somana 206—and nearby subsp. malaccensis (no collection). The photos are not to scale, but the orientation of the apical male flower clusters corresponds to their position in the inflorescence. Photos by Sasivimon C. Swangpol.

Figure 6.: Cluster hierarchy of the 186 accessions of Musa acuminata and nine outgroup accessions based on 16 morphological characters, Gower’s distance, and a UPGMA clustering algorithm. Note: The final rendered hierarchical clustering displays visible tips, as closely related terminal clusters with short cluster distances were collapsed into single representative nodes to ensure readability and prevent overlapping text.

Morphological deviations in Musa acuminata subspecies

In addition to the known variation in the shape of the leaf base, rachis orientation, and arrangement of the apical rachis bracts in the M. acuminata subspecies complex, several unusual features were revealed in the wild Thai populations studied. These included yellow rachis bracts in some populations of M. acuminata subsp. siamea and subsp. kraburiensis and a curious deviant inflorescence morphology in some populations of M. acuminata subsp. siamea.

A new form, f. byssina (Fig. 7A, B), of M. acuminata subsp. siamea was recently recorded in several locations in Chanthaburi (SE Thailand), Nakhon Ratchasima (E Thailand), and Kamphaeng Phet (N Thailand). It stands out by having yellow rachis bracts, in contrast to the typical red bracts of subsp. siamea. In Ranong (Peninsular Thailand), a new yellow-bracted mutant population of M. acuminata subsp. kraburiensis, named here f. luteola (Fig. 7C), was observed. It grew intermixed in a population of c. 50 individuals of the typical red-purple form scattered over approximately 4 km2 of a large open area. Animal visitation was observed in both the mutant and wild-type forms (Nilapaka, pers. obs.). Reduced levels or the absence of flavonols and anthocyanins have been reported in f. luteola, probably due to mutations in the genes encoding flavonol synthase, dihydroflavonol reductase, or other related enzymes in these pathways (Kitdamrongsont et al. 2008; Pothavorn et al. 2010). The yellow-bracted mutants are particularly common in some areas. This is probably a consequence of genetic drift and inbreeding within a critically small panmictic population due to deforestation and forest fragmentation.

Figure 7.: Morphological deviations in Musa acuminata subspecies. A, B. Inflorescences in the female and male phases, respectively, of the yellow-bracted mutant M. acuminata subsp. siamea f. byssina Swangpol & Athawongsa, f. nov., Swangpol & Somana 173, from Nakhon Ratchasima (NE Thailand); C. Male-phase inflorescence of the yellow-bracted mutant M. acuminata subsp. kraburiensis f. luteola Swangpol & W.Inta, f. nov., Swangpol & Somana 251, from Ranong (Peninsular Thailand); D. M. acuminata subsp. siamea f. nanakornii Swangpol, Chom. & Sukkaew, f. nov., showing aberrant lateral branch inflorescences during the male phase, Swangpol & Somana 448, from Nan (N Thailand). Photos by Sasivimon C. Swangpol.

An anomaly in the inflorescence architecture was observed in a mutant form of M. acuminata subsp. siamea, which is described here as forma nanakornii (Figs 7D, 8, 10). It is cultivated by several Hmong hill-tribe communities in the highlands of N and NE Thailand. The Thai name, ‘Roi Pli,’ refers to the occurrence of numerous lateral male-phase inflorescences produced on the main inflorescence axis. The unusual chandelier-like appearance of the inflorescence is caused by the primordia of the cincinni producing inflorescences instead of male flowers. Although this forma typically does not produce seeded fruit, an accession bearing a few seeded fruits has been observed (Swangpol & Somana 558).

Figure 8.: Developmental series of hands in the inflorescence of Musa acuminata subsp. siamea f. nanakornii (‘Kluai Roi Pli’). Images are arranged from older to younger developmental stages, proceeding from the upper left to the lower right. The uppermost left image represents a hand of female flowers. The subsequent hands show the abnormal male flowers, in which individual flowers are replaced by secondary male-phase inflorescences with various degrees of abnormality. This specific specimen, grown in Ratchaburi, Thailand, lacked an extended rachilla. Photos by Sasivimon C. Swangpol.