Citation: Bakalin, V.A.; Klimova, K.G.; Choi, S.S.; Nguyen, V.S. Once Again on the Distribution of Syzygiella (Adelanthaceae, Marchantiophyta) in Indochina. Diversity 2024, 16, 149. https://doi.org/10.3390/d16030149 Academic Editor: Michael Wink Received: 8 February 2024 Revised: 19 February 2024 Accepted: 21 February 2024 Published: 26 February 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). diversity Article Once Again on the Distribution of Syzygiella (Adelanthaceae, Marchantiophyta) in Indochina Vadim A. Bakalin 1, * , Ksenia G. Klimova 1 , Seung Se Choi 2, * and Van Sinh Nguyen 3 1 Laboratory of Cryptogamic Biota, Botanical Garden-Institute FEB RAS, Makovskogo Street 142, Vladivostok 690024, Russia; ksenia.g.klimova@mail.ru 2 Team of National Ecosystem Survey, National Institute of Ecology, Seocheon 33657, Republic of Korea 3 Institute of Ecology and Biological Resources, Graduate University of Science and Technology, Vietnam Academy of Science and Technology, Hanoi 10072, Vietnam; vansinh.nguyen@iebr.ac.vn * Correspondence: vabakalin@gmail.com (V.A.B.); hepaticae@jbnu.ac.kr (S.S.C.) Abstract: The distribution of known Syzygiella taxa in Indochina was reviewed. Currently, four species are known in Indochina: S. autumnalis, S. elongella, S. nipponica, and S. securifolia. This genus is re- ported for the first time in the flora of Cambodia, and S. securifolia is newly recorded for Vietnam. Herein, a description of oil bodies for S. securifolia is provided for the first time. A morphological description of the species and intravital photographs, as well as line-art illustrations, are provided along with the identification key to the Syzygiella taxa known in Indochina. A comparison of the climatic parameters of the collection sites for four known species showed that three of them occupy a relatively marginal position in the flora of Indochina as a whole and are known from colder biomes on the very northern edge of the peninsula. The locations of Syzygiella securifolia are scattered not only on the geographical map of Indochina but also on the bioclimatic scatterplot; these locations are likely an underestimation of the distribution of this taxon in Indochina, although it is generally rare worldwide. A comparison of lists of liverworts across the countries of Indochina will help identify groups of taxa for further targeted searches with the purpose of obtaining more comprehensive knowledge of the biodiversity of still poorly studied Indochina countries. Keywords: Syzygiella; Southeast Asia; Cambodia; Vietnam; liverworts; Hepaticae 1. Introduction Syzygiella is one of a few exceptions among liverwort genera that has undergone a change in scope based on molecular genetic research over the past 20 years. While several traditionally recognized large genera have been split into several smaller units [1–5], Syzygiella has expanded [6,7] in scope with the inclusion of a number of previously almost universally recognized genera, such as Cryptochila R. M. Schust., Jamesoniella (Spruce) F. Lees, and Roivainenia Pers. [8–10]. There are currently 33 species known within the genus Syzygiella [11]. The genus is sub- divided into 5 subgenera, the largest of which is Syzygiella, which includes 14 species [11]. The center of taxonomic diversity of the genus is in the Southern Hemisphere [8,12,13]. Only a few representatives of the genus are found in the Northern Hemisphere [14,15]. On the Southeast Asian mainland (Indochina Peninsula, Indochinese Floristic Province [16]), there are four representatives of the genus, and according to the literature, they include Syzygiella securtifolia, which is known in Thailand [17], as well as Syzygiella autumnalis (DC.) K. Feldberg et al., Syzygiella elongella (Taylor) K. Feldberg, Vá ˇ na, Hentschel et Heinrichs, and Syzygiella nipponica (S. Hatt.) K. Feldberg, Vá ˇ na, Hentschel et Heinrichs, which are known from the northernmost tip of North Vietnam [18–20]. It is worth mentioning that all three listed species are known from the Hoàng LiênSơn Range, and Syzygiella autumnalis is known from one additional location in Hà Giang (QuảnBạ district) of Vietnam. Diversity 2024, 16, 149. https://doi.org/10.3390/d16030149 https://www.mdpi.com/journal/diversity