Recessive male floret color for tracking gene flow in cultivated northern wild rice (Zizania palustris L.)

Abstract

Northern wild rice (NWR; Zizania palustris L.) is a wind‐pollinated, annual, aquatic grass that grows naturally in the Great Lakes region of the United States and Canada and is also cultivated in flooded paddies, predominantly in California and Minnesota. A better understanding of pollen‐mediated gene flow is needed within the species for both conservation and breeding efforts as cultivation occurs within the species’ natural range and spatially‐isolated, paddy structures are limited within breeding programs. Widely‐cited pollen travel research in NWR demonstrated that pollen could travel at least 3200 m. However, a population segregating for male sterility was used as the pollen recipient in the study and was determined to not be adequate for NWR pollen travel studies. Here, we present the characterization of a recessive white male floret (WMF) population in contrast to the dominant, purple male floret (PMF) color of cultivated NWR along with estimates of pollen‐mediated gene flow in a cultivated paddy setting. Studies conducted in 2018 and 2019 revealed that the primary amount of pollen‐mediated gene flow occurred within the first 7 m from the PMF donor source with no gene flow detected past 63 m. These results suggest that the likelihood of pollen‐mediated gene flow between cultivated NWR and natural stands remains low. We also identified a strong linkage between male floret, collar, and culm color. This study demonstrates that the WMF trait is an excellent candidate for use in pollen‐mediated gene flow studies in NWR.

Towards Stewardship of Wild Species and Their Domesticated Counterparts: A Case Study in Northern Wild Rice (Zizania palustrisL.)

“…Special efforts have been made to keep K2 from cross-pollinating with other breeding populations through spatial separation in small, isolated paddies. These findings along with those from Gietzel et al (2021), suggesting the majority of pollen-mediated gene flow occurs within 7 m from the pollen source, indicate that increasing spatial separation between Cultivated material would increase genetic differentiation and maintain the purity of populations in NWR breeding programs. Buffer zones between populations grown in close proximity (within the same paddy) and growing small populations in stock tanks that are spatially separated could help this effort.…”

“…These findings indicate that there is little to no gene flow between these two groups (with the exception of Phantom Lake), agreeing with previous diversity studies in NWR using different marker systems (Lu et al, 2005; Kahler et al, 2014; Diller et al, 2018). Limited gene flow in NWR is further supported by recent pollen-mediated gene flow studies that estimated the majority of NWR pollen only travels short distances (7 m) (Gietzel et al, 2021). Limited pollen travel has been reported for a close relative of NWR, Zizania texana , as well (Oxley et al, 2008).…”

Northern Wild Rice (Zizania palustris L.) breeding, genetics, and conservation

“…It is only possible to maintain a small number of germplasm stocks in the program, relative to other breeding programs, as research paddy space is limited and all germplasm must be grown yearly to maintain viable seed stocks. The program also maintains a recessive, white male floret population, used in Gietzel et al (2022) to track pollen-mediated gene flow, and a population derived from the single plant sequenced to assemble and annotate the first assembly of the NWR genome (Haas et al, 2021).…”

“…Each year, best plant selections are bulked and used to advance the next generation in the selection cycle (C1); a general unselected C0 bulk from the center of each plot is collected as well. As the majority of NWR pollen appears to travel only ∼20′ (Gietzel et al., 2022), population blocks are planted with 20′ buffer rows of the last cycle's general unselected bulk (C0) with the best plant selections (C1) in the center. This limits pollen‐mediated gene flow in the best plant selections from neighboring sources and maintains a high selection intensity, which while important for breeding, often results in small seed lots that are insufficient for the next year's seed needs.…”

“…The main variant staminate branch type is known as "bottlebrush," where branches remain parallel to the rachis like the wild type pistillate branching (Figure 2). This trait is also linked to male sterility, but this linkage can often be broken (Cregan, 2004;Gietzel et al, 2022). Cross-pollination of NWR most commonly occurs via wind.…”