Cuscuta denticulata subsp. autotetraploids

The tetraploids from the Walker Pass (Kern Co., CA) that were identified morphologically as C. denticulata have a D1 ribotype, not N2/D2 as in C. veatchii and C. psorothamnensis (Suppl. Fig. 1 and Fig. 1, solid circles). Also, trnL-F sequences belong to D1 haplotype, not D2 as in the allotetraploids. In the case of a hybrid origin of these tetraploid plants, the progenitors involved would have been from different populations compared to those involved in the origin of C. veatchii and C. psorothamnensis. Even though there is no topological incongruence between nrITS and plastid sequences for these individuals, a hybrid origin cannot be dismissed because concerted evolution might have occurred, removing the C. nevadensis arrays and keeping only those of C. denticulata. However, we argue that these plants are autotetraploids and not allopolyploids, for a number of reasons: morphological, karyological, and phylogenetic (Fig. 6). Morphologically, they have very different features compared to C. veatchii and C. psorothamnensis and are instead very similar to a typical C. denticulata (Fig. 4), except for bigger flowers and pollen, consistent with their polyploid condition. While the chromosome number is double, the chromosome size and karyotype are uniformly similar to those of C. denticulata (Fig. 3). Lack of a mix of larger and smaller chromosomes, as observed in hybrid polyploids (C. veatchii and C. psorothamnensis), is an additional line of evidence for autopolyploidy in these individuals of C. denticulata. Tetrasomic segregation and multivalent formation are normally expected in meiosis of autopolyploids, but unfortunately, we could not observe meiosis from limited samples in hand. Up to this point, the chromosome material has been obtained only from one locality (Walker Pass, Kern Co., CA) but we have studied herbarium specimens morphologically similar from Inyo Co., CA (Henrickson 17713), suggesting the existence of additional autotetraploid populations, which we hope to include in our future studies. Finally, our preliminary phylogenetic results on sequencing and cloning of multiple low copy nuclear pentatricopeptide repeat (PPR) genes (García et al. in preparation) also indicate the presence of subgenomes exclusively from C. denticulata in these tetraploids.

The autotetraploid cytotype occurs sympatric with the diploids but might show an incipient niche separation by growing exclusively on Ericameria (Compositae). As for allopolyploids, the host shift may provide an advantage for newly formed polyploids by reducing competition with adjacent diploid populations through the specialization to one host which is not at all (or is much less) frequented by the diploids. Our preliminary field observations suggest also a different phenology compared to the diploids, with the tetraploids having a later flowering time. Further research is necessary to document the extent of autopolyploidy in C. denticulata, including the study of more individuals from this and other morphologically similar populations together with their host range.