The green algal genus Ulva Linnaeus is a model organism for studying morphogenesis, development, and life cycles. Ulva is referred to as cryptic species; with limited external diagnostic features, yet genetically different. There have been limited molecular data available for seaweeds in the Mediterranean Coast of Egypt. This work aimed to delineate Ulva spp. by combining morphological examination and molecular-species-delimitation approaches including Assemble Species by Automatic Partitioning (ASAP, based on distance) and generalized mixed Yule-coalescent (GMYC, based on coalescence) using three genes; ITS, tufA, and rbcL. Species delimitation approaches have identified different partitions based on each gene, either by assigning similar species into a single partition using ASAP or by over splitting/lumping species using GMYC. Sequence-based identity comparisons of the investigated specimens with Ulva sequences available at NCBI revealed similarities with six Ulva species based on ITS, four species using tufA, and five species using rbcL. Additionally, five samples remained unidentified at the species level, suggesting a potential new species, which requires further comprehensive analysis. Based on phylogenetic analysis, the investigated species were placed at varying distances from different Ulva type specimens, as well as similar and dissimilar reference sequences. This variance can be attributed primarily to the absence of adequate type specimen sequences and/or the presence of misidentified entries within the NCBI database. Nucleotide diversity parameters revealed the highest sequence diversity of tufA, compared to ITS and rbcL. Combining morphological-and genetic analyses in Ulva spp. delimitation revealed the presence of U. fasciata, U. flexuosa,U. intestinalis, U. lactuca, U. linza, U. rigida, and Ulva sp. The current study enhances our comprehension of Ulva species diversity and underscores the significance of integrating molecular and morphological approaches for species delineation. This integration plays a pivotal role in addressing species complexes and precisely defining species boundaries.
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