Male dimorphism has been reported across different taxa, and is usually expressed as the coexistence of a larger morph with exaggerated male traits and a smaller one with reduced traits. The evolution and maintenance of male dimorphism are still poorly understood for several of the species in which it has been observed. Here, we analyse male dimorphism in several species of reptile parasitic nematodes of the genus Spauligodon, in which a major male morph (exaggerated morph), which presents the traditional male morphological traits reported for this taxon, coexists with a minor morph with reduced morphological traits (i.e. reduced genital papillae) resembling more closely the males of the sister genus Skrjabinodon than Spauligodon major males. Because of the level of uncertainty in the results of ancestral state reconstruction, it is unclear if the existence of male dimorphism in this group represents independent instances of convergent evolution or an ancestral trait lost multiple times. Also, although the number of major males per host was positively correlated with the number of females, the same did not hold true for minor males, whose presence was not associated with any other ecological factor. Nevertheless, the existence of male dimorphism in Spauligodon nematodes is tentatively interpreted as resulting from alternative reproductive tactics, with differences in presence and number of individuals as indicators of differences in fitness, with the lower numbers of minor males per host likely maintained by negative frequency-dependent selection.

Dataset MorphsDataset morphs: data used in the analyses of the factors related to MI and MA presence and abundance.Dataset_Morphs.csvSequence alignment of 18S ribosomal RNA (18S)Sequence alignment of 18S ribosomal RNA (18S) used for the phylogenetic analyses performed in MrBayes and PhyML (fasta format)Morphs18S.fastaSequence alignment of 28S ribosomal RNA (28S)Sequence alignment of 28S ribosomal RNA (28S) used for the phylogenetic analyses performed in MrBayes and PhyML (fasta format).Morphs28S.fastaSequence alignment of cytochrome oxidase subunit I (COI)Sequence alignment of cytochrome oxidase subunit I (COI) used for the phylogenetic analyses performed in MrBayes and PhyML (fasta format).MorphsCOI.fastaSequence alignment for the combined data-set of 28S and COISequence alignment for the combined data-set of 28S and COI used for the phylogenetic analyses performed in MrBayes used in ancestral state reconstruction (ASR).MorphsCOI_28S.fasta50% majority-rule consensus 28S and COI50% majority-rule consensus 28S and COI BI tree used in the ancestral state reconstruction (ASR) performed in MESQUITE.MorphsCOI_28S.nex.con.tre