Genetic evidence supports a distinct lineage of American crocodile (Crocodylus acutus) in the Greater Antilles

Draft assembly and annotation of the Cuban crocodile (Crocodylus rhombifer) genome

OBJECTIVES

The new data provide an important genomic resource for the Critically Endangered Cuban crocodile (Crocodylus rhombifer). Cuban crocodiles are restricted to the Zapata Swamp in southern Matanzas Province, Cuba, and readily hybridize with the widespread American crocodile (Crocodylus acutus) in areas of sympatry. The reported de novo assembly will contribute to studies of crocodylian evolutionary history and provide a resource for informing Cuban crocodile conservation.

DATA DESCRIPTION

The final 2.2 Gb draft genome for C. rhombifer consists of 41,387 scaffolds (contigs: N50 = 104.67 Kb; scaffold: N50-518.55 Kb). Benchmarking Universal Single-Copy Orthologs (BUSCO) identified 92.3% of the 3,354 genes in the vertebrata_odb10 database. Approximately 42% of the genome (960Mbp) comprises repeat elements. We predicted 30,138 unique protein-coding sequences (17,737 unique genes) in the genome assembly. Functional annotation found the top Gene Ontology annotations for Biological Processes, Molecular Function, and Cellular Component were regulation, protein, and intracellular, respectively. This assembly will support future macroevolutionary, conservation, and molecular studies of the Cuban crocodile.

Environmental DNA-based biomonitoring of Cuban Crocodylus and their accompanying vertebrate fauna from Zapata Swamp, Cuba

Crocodylians globally face considerable challenges, including population decline and extensive habitat modification. Close monitoring of crocodylian populations and their habitats is imperative for the timely detection of population trends, especially in response to management interventions. Here we use eDNA metabarcoding to identify the Critically Endangered Crocodylus rhombifer and the Vulnerable C. acutus, as well as vertebrate community diversity, in Cuba's Zapata Swamp. We tested four different primer sets, including those used previously in Crocodylus population genetic and phylogenetic research, for their efficiency at detecting crocodylian eDNA. We detected C. rhombifer eDNA in 11 out of 15 sampled locations within its historical geographic distribution. We found that data analyses using the VertCOI primers and the mBRAVE bioinformatics pipeline were the most effective molecular marker and pipeline combination for identifying this species from environmental samples. We also identified 55 vertebrate species in environmental samples across the four bioinformatics pipelines- ~ 85% known to be present in the Zapata ecosystem. Among them were eight species previously undetected in the area and eight alien species, including known predators of hatchling crocodiles (e.g., Clarias sp.) and egg predators (e.g., Mus musculus). This study highlights eDNA metabarcoding as a powerful tool for crocodylian biomonitoring within fragile and diverse ecosystems, particularly where fast, non-invasive methods permit detection in economically important areas and will lead to a better understanding of complex human-crocodile interactions and evaluate habitat suitability for potential reintroductions or recovery programs for threatened crocodylian species.

Use of continuous cranial shape variation in the identification of divergent crocodile species of the genus Mecistops

The discovery of cryptic biodiversity has blossomed under the advancements of genetic techniques, but species identification via morphology remains crucial to effective conservation efforts. In this study, we tested the use of continuous cranial shape variation in distinguishing the two living species of Mecistops: the West African slender-snouted crocodile (M. cataphractus) and the Central African slender-snouted crocodile (M. leptorhynchus). Using a combination of geometric morphometric characters and ratios of linear measurements, we identified statistically significant variation in cranial bone and overall skull shape of mature individuals that corroborates existing molecular and discrete morphological evidence for two distinct, extant species within Mecistops. Specifically, variation in the shape of the nasal appears particularly diagnostic, while ratios involving metrics of snout length to snout width at the premaxillary notch offer distinguishing features easily measured in the field. Because of the complementary results and applications of the morphometric and cranial ratio analyses, we argue that both methodologies remain relevant to species identification. Moreover, we recommend continued cooperation between geneticists and morphologists in diagnosing species of conservation concern.

Paleogenomics illuminates the evolutionary history of the extinct Holocene "horned" crocodile of Madagascar, Voay robustus

Ancient DNA is transforming our ability to reconstruct historical patterns and mechanisms shaping modern diversity and distributions. In particular, molecular data from extinct Holocene island faunas have revealed surprising biogeographic scenarios. Here, we recovered partial mitochondrial (mt) genomes for 1300-1400 year old specimens (n = 2) of the extinct "horned" crocodile, Voay robustus, collected from Holocene deposits in southwestern Madagascar. Phylogenetic analyses of partial mt genomes and tip-dated timetrees based on molecular, fossil, and stratigraphic data favor a sister group relationship between Voay and Crocodylus (true crocodiles). These well supported trees conflict with recent morphological systematic work that has consistently placed Voay within Osteolaeminae (dwarf crocodiles and kin) and provide evidence for likely homoplasy in crocodylian cranial anatomy and snout shape. The close relationship between Voay and Crocodylus lends additional context for understanding the biogeographic origins of these genera and refines competing hypotheses for the recent extinction of Voay from Madagascar.

Phylogenomics reveals novel relationships among Neotropical crocodiles (Crocodylus spp.)

Extant species in the order Crocodylia are remnants of an ancient lineage of large-bodied archosaur reptiles. Despite decades of systematic studies, phylogenetic relationships among members of the genus Crocodylus (true crocodiles) in the Neotropics are poorly understood. Here we estimated phylogenomic relationships among the four extant Crocodylus species in the Americas. Species-tree reconstructions using genotypic data from 17,538 SNPs collected for 33 individuals spanning six Crocodylus species (four ingroup and two outgroup) revealed novel relationships for all Neotropical species. For the first time, C. acutus, the American crocodile, was recovered as monophyletic when individuals from Antillean and continental populations were analyzed together. Our results also contradict previous inferences based on mitochondrial DNA data and a limited number of nuclear markers by robustly grouping Morelet's crocodile (C. moreletii) as the sister species to C. acutus, suggesting a novel phylogeographic hypothesis for the group. The present study punctuates the importance of using nuclear genome-wide information and representative sampling for resolving phylogenetic relationships, especially in broadly distributed species and those with complex evolutionary histories.

High levels of population genetic differentiation in the American crocodile (Crocodylus acutus)

The American crocodile (Crocodylus acutus) is a widely distributed species across coastal and brackish areas of the Neotropical region of the Americas and the Greater Antilles. Available information on patterns of genetic differentiation in C. acutus shows a complex structuring influenced by interspecific interactions (mainly hybridization) and anthropogenic actions (mostly historical hunting, recent poaching, habitat loss and fragmentation, and unintentional translocation of individuals). In this study, we used data on mitochondrial DNA control region and 11 nuclear polymorphic microsatellite loci to assess the degree of population structure of C. acutus in South America, North America, Central America and the Greater Antilles. We used traditional genetic differentiation indices, Bayesian clustering and multivariate methods to create a more comprehensive picture of the genetic relationships within the species across its range. Analyses of mtDNA and microsatellite loci show evidence of a strong population genetic structure in the American crocodile, with unique populations in each sampling locality. Our results support previous findings showing large degrees of genetic differentiation between the continental and the Greater Antillean C. acutus. We report three new haplotypes unique to Venezuela, which are considerably less distant from the Central and North American haplotypes than to the Greater Antillean ones. Our findings reveal genetic population differentiation between Cuban and Jamaican C. acutus and offer the first evidence of strong genetic differentiation among the populations of Greater Antillean C. acutus.