Definition of Eight Mulberry Species in the Genus <i>Morus</i> by Internal Transcribed Spacer-Based Phylogeny

<div><p>Mulberry, belonging to the order Rosales, family Moraceae, and genus <i>Morus</i>, has received attention because of both its economic and medicinal value, as well as for its important ecological function. The genus <i>Morus</i> has a worldwide distribution, however, its taxonomy remains complex and disputed. Many studies have attempted to classify <i>Morus</i> species, resulting in varied numbers of designated <i>Morus</i> spp. To address this issue, we used information from internal transcribed spacer (ITS) genetic sequences to study the taxonomy of all the members of generally accepted genus <i>Morus</i>. We found that intraspecific 5.8S rRNA sequences were identical but that interspecific 5.8S sequences were diverse. <i>M</i>. <i>alba</i> and <i>M</i>. <i>notabilis</i> showed the shortest (215 bp) and the longest (233 bp) ITS1 sequence length, respectively. With the completion of the mulberry genome, we could identify single nucleotide polymorphisms within the ITS locus in the <i>M</i>. <i>notabilis</i> genome. From reconstruction of a phylogenetic tree based on the complete ITS data, we propose that the <i>Morus</i> genus should be classified into eight species, including <i>M</i>. <i>alba</i>, <i>M</i>. <i>nigra</i>, <i>M</i>. <i>notabilis</i>, <i>M</i>. <i>serrata</i>, <i>M</i>. <i>celtidifolia</i>, <i>M</i>. <i>insignis</i>, <i>M</i>. <i>rubra</i>, and <i>M</i>. <i>mesozygia</i>. Furthermore, the classification of the ITS sequences of known interspecific hybrid clones into both paternal and maternal clades indicated that ITS variation was sufficient to distinguish interspecific hybrids in the genus <i>Morus</i>.</p>