Abstract
BackgroundDuodichogamy is the rarest form of dichogamy in angiosperms, which is characterized by flowering in the sequence of male→female→male. Disentangling factors promoting duodichogamy require the discovery of more duodichogamous species in angiosperms. However, extremely limited information on duodichogamous species makes it difficult to make general conclusions. Given the inflorescence morphology and flowering characteristics, the Meliaceae family is highly likely to contain duodichogamous species.MethodsWe selected 48 individuals from 20 populations in Korea and investigated their flower morphology, arrangement of flowers by sexual condition within inflorescences, and flowering phases and duration of male and female flowers of Toona sinensis (Meliaceae) for 5 years (2011–2015) to determine if the species shows duodichogamous flowering.ResultsToona sinensis belonging to Meliaceae possessed functionally unisexual flowers with rudimentary parts of the opposite sex. The floral organs in male were larger than those in female, except for ovary length and width. In dichasium, male flowers were observed on primary or lateral branches, whereas female flowers were borne only on lateral branches. Overall, individuals from six different populations flowered in the male→female→male sequence, thereby male is blooming far longer than female flowers at the level of individual trees (male vs. female = 17–20 days vs. 2–4 days).ConclusionsThis is the first study to report a duodichogamously flowering species, T. sinensis, within Meliaceae. Several flowering characteristics observed from T. sinensis may be important clues used to discover additional duodichogamous Meliaceae species. Short flowering period and relatively small number of female flowers, which is analogous to reduced ovule numbers observed in other duodichogamous species, may intensify male–male competition in T. sinensis. This study contributed to narrowing down potential candidates of duodichogamy based on their geographic distributions and flowering time.
Highlights
Duodichogamy is the rarest form of dichogamy in angiosperms, which is characterized by flowering in the sequence of male→female→male
We investigated the type of dichogamy associated with T. sinensis to answer the following questions: (1) Does morphology differ between male and female T. sinensis flowers? (2) How are male and female flowers arranged within inflorescences? (3) Is the flowering pattern consistent with duodichogamy? To answer these questions, we observed flower morphology, the arrangement of flowers based on sexual condition within inflorescences, and flowering phases and duration of male and female flowers
Male and female flower characteristics We found several common characteristics between male and female T. sinensis flowers
Summary
Duodichogamy is the rarest form of dichogamy in angiosperms, which is characterized by flowering in the sequence of male→female→male. Disentangling factors promoting duodichogamy require the discovery of more duodichogamous species in angiosperms. Dichogamy is a temporal flowering system in which male and female reproductive organs mature at different times within a single flower or across multiple flowers. Protandry refers to instances when the maturation of the male reproductive organ precedes that of the female. Synchronous dichogamy is further subdivided into one-cycle dichogamy, duodichogamy, multi-cycle dichogamy, and heterodichogamy, depending on the sequence and cyclicity of maturation of male and female flowers (unisexual) or each reproductive organ (bisexual) (Lloyd and Webb 1986). One-cycle dichogamy is flowering in the sequence male→female or female→male, duodichogamy is flowering in one-and-a-half cycles, usually in the sequence (male→female→male), and multi-cycle dichogamy has repeated flowering cycles alternating between male and female. Heterodichogamy has two morphs (protandrous and protogynous morphs) that flower synchronously within unisexual or bisexual populations
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