Host range, host specificity, regional host preferences and genetic variability of <i>Korthalsella</i> Tiegh. (Viscaceae) mistletoes in New Zealand

The distribution of parasitic plants is governed by the availability and distribution of suitable host species. Here we report the distributions, host associations and genetic variability of the three endemic New Zealand species of <i>Korthalsella</i> (Viscaceae) mistletoes. These species differ in their primary host and in the breadth of secondary and tertiary hosts. <i>Korthalsella salicornioides</i> is the most host-specific, with 96% of the total records from the primary (<i>Leptospermum scoparium</i> s.l. [Myrtaceae]) and secondary (<i>Kunzea robusta</i> [Myrtaceae]) hosts, but 26 host taxa (six genera in five families) recorded overall. <i>Korthalsella clavata</i> and <i>K. lindsayi</i> are more generalist species, parasitising 44 taxa (19 genera in 14 families) and 42 taxa (13 genera in 11 families), respectively. For <i>K. clavata</i>, <i>Coprosma propinqua</i> (Rubiaceae) and <i>C. dumosa</i> are the primary and secondary hosts, respectively, representing 66% of the total records. For <i>K. lindsayi, Melicope simplex</i> (Rutaceae) is the primary host (26% of records), whereas <i>Coprosma crassifolia</i>, <i>Lophomyrtus obcordata</i> (Myrtaceae), <i>Myrsine australis</i> (Primulaceae) and <i>M. divaricata</i> are secondary hosts. The only overlap among hosts is at the tertiary level, with <i>K. clavata</i> and <i>K. lindsayi</i> sharing <i>Coprosma rigida</i> and <i>C. virescens</i>, and among hosts that are rarely parasitised. <i>Korthalsella salicornioides</i> is the most widely distributed species occurring widely on the North and South Islands, while <i>K. clavata</i> is rare in the North Island and <i>K. lindsayi</i> is rare in the western regions of the South Island. Comparative sequencing of nuclear internal transcribed spacers and plastid <i>trnQ–rps16</i> for the three mistletoes revealed that within-species genetic structure is geographic rather than host-based. Additionally, some plastid haplotypes and ITS sequence types shared between <i>K. clavata</i> and <i>K. lindsayi</i> may result from local hybridisation and introgression between the two species in a few locations.