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 Finding the genes could enable new “genetic control” methods for
the weeds, which, in many places, no longer respond to
herbicides.
“If we knew which genes control maleness and we could make those
genes proliferate within the population, every plant in the
field would be a male after a few generations, and
theoretically, the population would crash,” says Pat Tranel,
professor and associate head in the Department of Crop Sciences
at the University of Illinois and lead author on a study in New
Phytologist.
Tranel and his colleagues had previously identified molecular
markers associated with the male genomic region. After
sequencing male genomes for both species, the researchers were
able to use those markers to zero in on the male-specific
region. Now, they are within 120 to 150 genes of finding their
target.
“We're confident most of those 120 or so genes are probably
doing nothing. It's just stuff that's accumulated in that region
of the genome,” Tranel says. “If I had to guess, I’d say maybe
10 of them are actually doing something relevant.”
Narrowing down the genes related to gender in these weeds could
have practical value for control, but the study also sheds light
on the phenomenon of dioecy – male and female sexual organs on
separate individuals – more generally. The vast majority of
animals are dioecious, but it’s rare in plants. More than 90% of
flowering plants have both sexual organs on the same individual,
and often within the same flower.
Waterhemp and Palmer amaranth, however, are dioecious.
Dioecy means it’s impossible for a plant to self-pollinate;
instead, female gametes must be fertilized by male pollen from
another plant. That’s a good thing for ensuring genetic
diversity in a population. And it’s likely what has made
waterhemp and Palmer amaranth so successful at evading the
detrimental effects of multiple herbicides.
“To date, waterhemp and Palmer amaranth have evolved resistance
to herbicides spanning seven and eight modes of action,
respectively. Dioecious reproduction results in all these
resistance traits being mixed and matched within individuals.
This mixing has allowed populations of both species to combine
multiple herbicide resistances, leaving producers with few
effective herbicide choices,” Tranel says.
Understanding the rare phenomenon of dioecy in
plants can help scientists piece together how traits are
inherited from each parent, and to understand how the phenomenon
evolves.
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Unlike in animals, in which dioecy is thought to have
evolved just once, scientists believe dioecy in plants has evolved
numerous times. And, according to Tranel’s study, it appears to have
evolved independently in waterhemp and Palmer amaranth, two very
closely related species.
“I'm not ready to say we absolutely know they evolved separately,
but all the information we found supports that idea. Only two of the
120-150 genes were similar to each other across the two species,”
Tranel says.
One of those shared genes, Florigen, helps plants respond to day
length by initiating flowering. Tranel doesn’t know yet whether it
determines the gender of flowers, but he’s intrigued that it showed
up in the male-specific Y region for both species.
“We don’t know for sure, but maybe it’s involved with males
flowering earlier than females. That could be advantageous to males
because then they’d be shedding pollen when the first females become
receptive. So if, in fact, Palmer and waterhemp really did evolve
dioecy separately, but both acquired this Florigen gene for a
fitness advantage, that would be a cool example of parallel
evolution.”
Tranel hopes to narrow down the male-specific Y region in both
species even further to isolate the genes that determine maleness.
There’s no guarantee a genetic control solution will be developed
once those genes are identified – Tranel would likely need to
attract industry partners for that – but having such a tool is not
as far off as it once was.
The article, “Male-specific Y-chromosomal regions in waterhemp (Amaranthus
tuberculatus) and Palmer amaranth (Amaranthus palmeri),” is
published in New Phytologist [DOI: 10.1111/nph.17108]. Authors
include Jacob Montgomery, Darci Giacomini, and Pat Tranel of the
University of Illinois, and Detlef Weigel of the Max Planck
Institute for Developmental Biology. The project was supported by
the USDA National Institute of Food and Agriculture and the Max
Planck Society.
The Department of Crop Sciences is in the College of Agricultural,
Consumer and Environmental Sciences at the University of Illinois.
[Source: Pat Tranel
News writer: Lauren Quinn] |
