This study aimed to understand the research facets of the genus Melia due to its increasing economic importance. Scientific publications are regarded as the most reliable source of information on research topics, and these are typically accessed using keyword searches in databases, including Google Scholar. Scientific production of the genus has increased linearly over the years, with some fluctuations (Fig. 1). This genus has been studied in more than 80 countries, indicating its widespread geographical distribution and interest.
Typically, scientific names are the vital links connecting the findings of researchers in different disciplines. Unfortunately, variations in the spelling of botanical names can result in an article not being found. In our study, we came across different specific epithet spellings for the genus Melia. For instance, the specific epithet of Melia azedarach was misspelled as azedericta, azederach, azedarch, azediarach, azedarrah, azedarack, etc. Moreover, there were more than 55 species listed under the genus Melia in the International Plant Names Index (https://www.ipni.org/), as well as approximately 57 species listed in the WFO Plant List (https://wfoplantlist.org/plant-list). However, only three species are listed as accepted: M. dubia, M. azedarach and M. volkensii. However, in our research, we could see the predominant usage of the synonyms of M. azedarach as different species. For instance, M. toosendan, a synonym of M. azedarach, is regarded as a separate individual species in China (Yu et al. 2004; Liao et al. 2016). Similarly, M. composita is a synonym for M. dubia (Sharma et al. 2018), but it is sometimes treated as an independent species in India (Singh et al. 2017). Even M. dubia was regarded as a synonym for M. azedarach (Koul et al. 2002) until Sivaraj et al. (2018) proved them to be two different independent species. These misconceptions and renamings may not be unique to the genus Melia. The issue raises serious concern because some Melia species are important in ethnobotanical and ethnoveterinary practice, while others are also reported to be toxic. (Tringali 2000; Ward et al. 2008). This calls for work to clarify the taxonomy of the genus.
Scientific names allow us to cross-reference information about organisms globally. However, variations in the spelling of scientific names greatly diminish the ability to interconnect data. Such variations may include abbreviations, annotations, misspellings, etc. (Tringali 2000). Our concerns are more related to variations in scientific names that can hamper the use of approaches such as text mining, systematic review, metanalysis, and research weaving (Nakagawa et al. 2019; Asmussen and Møller 2019).
We concluded that the following species are the most researched species in the genus Melia based on the number of citations: M. azedarach, M. toosendan, Melia dubia, and M. composita. The trend topics also pointed out a similar list of species (Fig. 2). We also found papers on M. indica, M. birmanica, and M. excelsa on topics such as metabolite constituents and wood properties. However, these publications were not cited. Most of them were published before 2000, except for Iswanto et al. (2010), which dealt with wood properties of Melia excelsa. It seems that Melia excelsa is a synonym for Azadirachta excelsa, but this synonym is rarely used in scientific literature.
The relevant main topics and their changes over time with respect to the genus Melia are assessed based on trend topics (Fig. 2), a thematic evolution map (Fig. 3), and a strategic diagram (Fig. 4). These were based on coword analysis performed with the Louvain algorithm due to its good performance in terms of modularity and processing time (Blondel et al. 2008). For a typical thematic evolution map, the total publications will be divided into subperiods. In our analysis, given the vast differences in the levels of production over time, the overall publications were grouped into four major timelines such that the average number of publications in a single time period was ~ 300 scientific papers. Combined insights from science mapping were used to measure the development of the specific research theme in a conceptual way for a specific time period.
Earlier research on the genus was focused on its medicinal and repellent properties. For instance, one of the oldest publications on the genus Melia is on the exploration of the repellent properties of its two species (Lavie et al. 1967). Melia azedarach leaf extract might have antiviral properties, which was studied in the late 1990s (Andrei et al. 1990). The possibility of using it for controlling specific insects, such as the diamondback moth (Plutella xylostella), triatomine bugs, the variegated cutworm (Peridroma saucia), the maize weevil (Sitophilus zeamais), manu, other insects, and some fungal diseases, was also explored. These initial investigations on insecticidal and antifeedant activity encouraged researchers to explore more at the biochemical and genome levels in more recent years (Liao et al. 2022) (Fig. 2). Melia azedarach and Azadirachta indica have both been well-studied secondary metabolites, and both are in the Meliaceae family. Wood usage was not a focus of research for the genus Melia until the early 2000s (Trevisan et al. 2007) (Fig. 4d). In the last few years, the entire focus of Melia research has been on its usage as a potential wood-yielding species. This has led to research on its germination, reproduction, growth and biomass production. We hypothesize that the increased wood demand and trade has urged stakeholders to explore suitable alternative species for wood production (FAO 2020). In addition, researchers have worked on some fast-growing trees, such as Eucalyptus, Popular, and Casuarina, and have stumbled upon the fast-growing Melia and ventured into this genus. Melia dubia and M. composita have proven wood qualities, unlike M. azedarach, which is regarded as an invasive plant in the USA and some parts of Africa (Ding et al. 2006; Voigt et al. 2011). Nevertheless, M. azedarach is reported to be highly suitable for paper and pulp (Megra et al. 2022). Recently, there has been increased research on the green synthesis of nanoparticles related to Melia, which hints at new potential for this genus.
