- Heterosis, or hybrid vigour, occurs when the progeny of two diverse varieties of a species exhibit greater yield, growth rates and fertility than either parent.
- It’s a contributing factor to the superior rates of genetic gain observed in maize breeding programmes compared with perennial ryegrass.
- Capturing heterosis in perennial ryegrass (an outbreeding, self-incompatible species) has so far been difficult.
- Recent advances in genetic mapping mean plant breeders can now select perennial ryegrass lines suitable for hybrid breeding, based on a method first proposed in the 1970s.
- While this breeding method is at the early stage of development, initial results are promising.
DairyNZ is investing in the development of new hybrid perennial ryegrass cultivars as part of a research programme led by Australian organisation DairyBio, a joint venture between Agriculture Victoria, Dairy Australia and the Gardner Foundation. This will allow breeders to exploit hybrid vigour in perennial ryegrass, which hasn’t been possible until now. Maize breeders have been successfully exploiting hybrid vigour in their breeding programmes for over 70 years – which has contributed to maize’s superior rates of genetic gain compared with perennial ryegrass. Initial field trial evaluations of the new cultivars in New Zealand and Victoria, Australia, are promising.
Perennial ryegrass: New Zealand history
Following perennial ryegrass’s introduction to New Zealand in the 1800s, plant breeders identified local ecotypes (local populations that had adapted to their environmental conditions), which exhibited superior performance. One particularly persistent ecotype was identified in Hawke’s Bay, from which a strain with superior winter and spring growth was selected. This strain was certified in 1934 and later named Grasslands Ruanui.
Further progress was made when the Mangere ecotype of perennial ryegrass was identified on the farm of Trevor Ellet in South Auckland. This out-yielded Grasslands Ruanui in on-farm trials1. Many important cultivars have been developed from the Mangere ecotype, including, Nui, Yatsyn and Bronsyn1.
More recently, plant breeders incorporated genetic material from northwest Spain into their breeding programmes, leading to the development of winter-active, late flowering cultivars, e.g. Bealey2.
Gains in dry matter yield
Estimates of genetic gain in annual dry matter (DM) yield for New Zealand perennial ryegrass cultivars have ranged from 0.25 to 0.76 percent per annum, with an average estimate of 0.5 percent per annum1 3. Similar values have been reported from Europe4. One study showed that genetic gains in the DM yield of Australian and New Zealand bred cultivars was limited prior to 1990, but since 1990, consistent genetic gains of 0.76 percent per annum have occurred3.
These gains can be considered quite modest compared with the genetic gains delivered by maize improvement programmes (for example, gains of 2.6 percent per annum in machine-harvestable grain yield have been reported5). Researchers have cited lower levels of investment and longer breeding cycles among the reasons. Another important constraint has been an inability to exploit hybrid vigour (‘heterosis’) effectively in commercial perennial ryegrass breeding programmes.
Heterosis, or hybrid vigour
Heterosis occurs when the progeny of two diverse varieties of a species, or crosses between species, exhibit greater yield, speed of development, and fertility than either parent6. New Zealand dairy farmers are already familiar with this concept – the common practice of cross-breeding dairy cattle can result in an animal that’s more productive than either of the parental breeds7.
Heterosis has also been captured in many agricultural plant breeding programmes. For example, hybrid breeding has been successfully used in maize breeding programmes since the 1930s and has led to significant gains in yield (Figure 1)8.
Suitability for hybrid breeding: maize vs. ryegrass
The commercial production of hybrid plants requires two important steps. Firstly, inbred lines are created to reduce genetic variation, by self-fertilising individual plants through successive generations. This ensures the offspring of these lines are predictable and uniform.
The inbred lines are then mated (cross-pollinated). In the field, breeders ensure that cross-pollination between the inbred parent lines occurs (to avoid further self fertilisation). The resultant ‘F1’ hybrid plants display significant levels of heterosis on-farm.
Maize lends itself well to hybrid breeding because it’s possible to self-fertilise plants and create inbred-parent lines. Also, cross-pollination in the field can be easily achieved by mechanically removing the male flower (de-tasselling) from one of the parent lines.
Because perennial ryegrass is an outbreeding, self-incompatible species (see next section), so far it has been difficult to effectively exploit heterosis in perennial ryegrass breeding programmes. It’s not that heterosis doesn’t occur in current commercial ryegrass breeding programmes. The problem is, it’s not captured in the seed that farmers purchase. That’s because the initial hybrid vigour present in the small number of plants initially crossed (five to 10 plants) is lost as their offspring plants are back-crossed and crossed again.
Self-incompatibility in ryegrass
Self-incompatibility is a mechanism employed by some plants to maximise cross-pollination and restrict self-fertilisation and inbreeding. In nature, this mechanism ensures genetic diversity and limits the loss of vigour associated with inbreeding. This characteristic naturally causes problems for perennial ryegrass breeders who want to create inbred parent lines for use in a hybrid breeding programme.
However, genetic diversity occurs in the mechanisms controlling self-incompatibility in perennial ryegrass. This forms a basis for a hybrid breeding method known as the self incompatibility method9. This method relies on the self-incompatibility system in ryegrass not being fully effective, which means some specific parental lines can be inbred.
This approach turns the traditional breeding process on its head, so that the final cross is carried out on a large scale (tens of hectares) to release hybrid vigour. The lead-up to this involves several years of inbreeding using two selected parental lines. This ensures enough seed can be generated from each line so they can be cross-pollinated in the final seed crop to produce commercial quantities of seed. That seed then goes directly into farmers paddocks, carrying a high level of heterosis.
There’s always a catch
There is still a catch in this method that breeders need to overcome. In this case, it’s managing the multiple crosses of the parental inbred lines to generate the amount of seed needed to grow the final crop. To successfully produce hybrids, crossing of the inbred lines must be controlled to ensure the resultant plants are not a result of self fertilisation (or further inbreeding). This isn’t so straightforward with an outbreeding, wind-pollinated species such as perennial ryegrass.
Controlled pair-crossing of perennial ryegrass lines in the field isn’t realistic, as fertilisation occurs via a pollen cloud from numerous surrounding plants10. While the self incompatibility mechanism generally ensures that perennial ryegrass cross-pollinates, the inbred lines, by their nature, can potentially inbreed further11, leading to a reduction in the proportion of F1 hybrid seed.
In order to generate a high proportion of F1 hybrid seed, the inbred parental lines must cross-pollinate. Therefore, the breeding lines must be selected to ensure the compatibility between the two inbred parental lines (i.e. ability to cross-pollinate) is greater than the compatibility within each inbred parental line (inbreeding). (See Figure 2).
Gene marker breakthrough
Predictive genetic markers (segments of DNA that can be used to track genes) for the genes controlling self-incompatibility in perennial ryegrass have now been identified. Using this technology, breeders can selectively target parental lines to achieve the compatibility targets noted above.
Analysis by researchers indicates that breeding schemes based on the self-incompatibility method, when combined with the use of genetic mapping to target specific breeding lines, has the potential to generate seed lines with an 83 percent proportion of F1 hybrids10.
To date, there has been little evaluation of the F1 hybrid plants generated by this breeding method. To be commercially viable and of value to famers, the breeding method must generate F1 hybrids that are superior to the better parent (high-parent heterosis)12, and indeed, superior to the elite ryegrass cultivars commercially available now.
Glasshouse study – DairyNZ
Recently DairyNZ funded a study by Louise Brok, a DairyNZ and Massey University Masters student, which evaluated F1 hybrid plants developed using the self incompatibility breeding method. The objective of her work was to detect early proof of increased yield performance in F1 hybrid plants. Due to limited seed availability, Louise’s experiment was conducted at an individual plant scale in a glasshouse. F1 hybrid plants produced by the self-incompatibility breeding method displayed mid-parent heterosis, i.e. the F1 hybrid was superior to the parental average.
This provides evidence that the proposed breeding method can successfully exploit heterosis. In addition, high-parent heterosis was detected, which indicates the breeding method has the potential to produce plants that out-yield current commercially-available cultivars.
More data from field trials will be required to corroborate results from Louise Brok’s glasshouse experiments. In addition, further work will be required to identify parent lines most suitable for the breeding method which will maximise heterosis, and DM yield on-farm. New Zealand plant breeding companies are testing hybrid perennial ryegrass cultivars developed using the methods described above in the field, and seed could be commercially available within five years.
- Lee, J. M., C. Matthew, E. R. Thom, and D. F. Chapman. 2012. Perennial ryegrass breeding in New Zealand: a dairy industry perspective. Crop and Pasture Science 63:107-127.
- Stewart, A. V. 2006. Genetic origins of perennial ryegrass (Lolium perenne) for New Zealand pastures. Pages 11-20 in Proceedings of the 13th Australasian plant breeding conference (Breeding for success: diversity in action, Ed. C. F. Mercer). New Zealand Grassland Association: Christchurch, New Zealand.
- Harmer, M., A. V. Stewart, and D. R. Woodfield. 2016. Genetic gain in perennial ryegrass forage yield in Australia and New Zealand. Journal of New Zealand Grasslands 78:133-138.
- McDonagh, J., M. O’Donovan, M. McEvoy, and T. J. Gilliland. 2016. Genetic gain in perennial ryegrass (Lolium perenne) varieties 1973 to 2013. Euphytica 212:187-199.
- Tollenaar, M. 1989. Genetic improvement in grain yield of commercial maize hybrids grown in Ontario from 1959 to 1988. Crop Science 29:1365-1371.
- Birchler, J. A., H. Yao, S. Chudalayandi, D. Vaiman, and R. A. Veitia. 2010. Heterosis. The Plant Cell 22:2105-2112.
- Lopez Villalobos, N. 1998. Effects of crossbreeding and selection on the productivity and profitability of the New Zealand dairy industry. (Thesis). Massey University.
- Duvick, D. 2005. Genetic progress in yield of United States maize (Zea mays L.). Maydica 50:193-202.
- England, F. 1974. The use of incompatibility for the production of F1 hybrids in forage grasses. Heredity 32:183-188.
- Pembleton, L., H. Shinozuka, J. Wang, G. Spangenberg, J. W. Forster, and N. Cogan. 2015. Design of an F1 hybrid breeding strategy for ryegrasses based on selection of self-incompatibility locus-specific alleles. Frontiers in Plant Science 6:764.
- Herridge, R. P., R. C. Macknight, and L. R. Brownfield. 2019. Prospects for F1 hybrid production in ryegrass. New Zealand Journal of Agricultural Research doi: 1080/00288233.2018.1559867
- Barret, B., M. Turner, T. Lyons, M. Rolston, and H. Easton. 2010. Evaluation of semi-hybrid perennial ryegrass populations. Pages 11-16 in Proceedings of the New Zealand Grassland Association. New Zealand Grassland Association.
This article was originally published in Technical Series September 2019