Ascaris from Humans and Pigs Appear to Be Reproductively Isolated Species

The taxonomic status of Ascaris lumbricoides and A. suum is contentious: shall they be considered a single species? Yes, as argued by Leles et al. [1], and most recently debated through opposing views by Alves et al. [2] and Betson et al. [3]. Most taxonomic literature refers to two species, and results from investigations of the nuclear DNA of Ascaris isolates of human and pig origin suggest that they are reproductively isolated and thereby fulfill the biological species definition. In our opinion, two species should therefore still be recognized until convincing evidence is presented to justify a reclassification. And yes, a number of studies have compared Ascaris isolates from pigs and humans and have identified close similarities in mitochondrial [4–6] and single nuclear markers (such as internal transcribed spacer [ITS] [5,7] and 18S rRNA [8]), including shared genotypes and haplotypes. It has been clearly established that Ascaris can cross infect hosts experimentally and in natural settings, as reviewed by Nejsum et al. [9].