Labronema species

Álvarez-Ortega, Sergio, 2025, Advancing the understanding of morphological and molecular diversity of the genus Labronema Thorne, 1939 (Nematoda: Dorylaimida) with new insights into its molecular phylogeny, Zoologischer Anzeiger 315 (8), pp. 105-123 : 122

4.3. Phylogenetic relationships of the Labronema species

The molecular phylogenetic trees based on the D2-D3 expansion segments of the 28S rRNA gene ( Fig. 3 ) and partial 18S rRNA gene ( Fig. 4 ) confirm that the genus Labronema is not monophyletic. Instead, the analyses reveal that Labronema sequences cluster into at least three distinct species groups, primarily evidenced by the D2-D3 of 28S rRNA tree and reflected similarly in the partial 18S rRNA tree. Given that the 18S rRNA gene is a more conserved DNA region with lower resolution for genus and species differentiation within the suborder Dorylaimina ( Holterman et al., 2008; van Megen et al., 2009; Alvarez-Ortega´& Pena-Santiago ˜, 2019), the D2-D3 of 28S rRNA tree has been used as the reference for phylogenetic relationships in this study.

The three obtained groups are designated as Labronema Group-1 (L1), Group-2 (L2), and Group-3 (L3). Group L1, which is considered to represent the true phylogenetic position of Labronema sense stricto , includes the type species L. ferox as well as L. stechlinensis . Members of this group are characterized by some shared morphological traits, such as a relatively large body size (averaging ≥3.0 mm), a complex female reproductive system featuring an elongated uterus with an intermediate pars musculosa, and a pattern of progressive tail length reduction during postembryonic development. This group may also incorporate other Labronema species that exhibit similar morphological patterns, including L. hyalinum and L. thornei , which share these defining characteristics and may thus align phylogenetically within Group L1.

In contrast, Group L2 consists of L. bidoupense , L. porosum , and L. vulvalatpilatum , species that are generally smaller, with an average body length of ≤ 2.5 mm. Group L3 includes L. montanum and L. octodurensis , distinguished by a distinctively angular lip region with low perioral liplets, also a generally smaller body size (typically ≤ 2.1 mm), and a post-equatorial vulva position (V = 57–60). While L. montanum and L. octodurensis group together in L3 within the D2-D 3 28S rRNA phylogenetic tree, they do not form a single clade in the partial 18S rRNA tree, which shows a less-supported topology (L3a and L3b). Nonetheless, their closer phylogenetic proximity to each other relative to other Labronema species in the 18S tree aligns with the broader grouping pattern observed in the 28S rRNA analysis. These two groups (L2 and L3) may potentially include additional Labronema species of similar, smaller size. Identifying additional species in these groups could help reveal more precise morphological traits, which would aid in the precise redefinition of distinguishing characteristics for each Labronema species group.

In this phylogenetic analysis, the trees incorporate molecular data from only seven of the 49 currently valid Labronema species. Despite this limited numbers of sequenced species, the results clearly indicate that Labronema is polyphyletic, comprising at least three distinct species groups. However, elucidating the internal phylogenetic relationships within the genus remains challenging due to the absence of clear autapomorphies among the identified Labronema groups, which complicates efforts to delineate a truly monophyletic taxon.

To address these phylogenetic ambiguities, further molecular characterization of additional Labronema species is essential, ideally combined with detailed morphological studies in an integrative approach. Expanding the genetic data to include more representatives of the genus, coupled with refined morphological analyses, will be crucial for accurately reclassifying Labronema and establishing monophyletic groups within it. Continued research will likely clarify these evolutionary relationships, leading to a taxonomic restructuring that more precisely reflects the lineage diversification within this complex genus.

However, it is also important to emphasize that this study represents a further step toward elucidating the internal evolutionary relationships of Labronema . By identifying distinct phylogenetic groups, this work lays the foundation for a more refined and comprehensive understanding of the genus, providing direction for future investigations into Labronema systematics and contributing to broader insights into dorylaimid evolution.

Finally, although additional sequences from another DNA marker, the partial ITS rDNA gene, were obtained for the four studied Labronema species in this study, the limited number of available dorylaimid sequences for this gene region in databases (such as GenBank) currently hinders a comprehensive phylogenetic analysis. It is likely that incorporating a phylogenetic analysis based on the partial ITS rDNA gene sequences could provide further insights into the internal evolutionary relationships within Labronema . However, this will only be achievable once the molecular characterization of dorylaimid species routinely includes data from this DNA region.

Figures

Fig. 3. Phylogenetic positions of Labronema species within some representatives of the suborder Dorylaimina as inferred from Bayesian analysis of sequences of the D2-D3 expansion segments of the 28S rRNA gene using GTR + I + G model of DNA evolution. Posterior probability values more than 70 % are given on appropriate clades. New sequence is indicated in bold. The notation Genus name + spp. indicates that this clade includes multiple species within the specified genus.

Fig. 4. Phylogenetic positions of Labronema species within some representatives of the suborder Dorylaimina as inferred from Bayesian analysis of sequences of the 18S rRNA gene using GTR + I + G model of DNA evolution. Posterior probability values more than 70 % are given on appropriate clades. New sequence is indicated in bold. The notation Genus name + spp. indicates that this clade includes multiple species within the specified genus.