Transcript Document

AG Genetics
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
IN THE BEGINNING…
To best interpret the genetics behind our beloved Atlantic Giant, we
must first understand the basics of plant reproduction. What follows is a
very basic tutorial.
It is important at this time to properly classify the Atlantic Giant
genetically. It is of the genus Cucurbita, a genus reserved for squash, and
of the species maxima. Other members of this species include the
Banana, Cinderella, and Burpee’s Prizewinner Hybrid. By definition,
it is not a true pumpkin like the Howden’s Field, Connecticut Field, or
Sorcerer pumpkins and cannot be crossed with them. These pumpkins
are of the species Cucurbita pepo. Furthermore, the difference between
today’s Atlantic Giant pumpkin and the Show King giant squash is
genetically negligible as these appear to differ only in color. One might
look at a true Show King squash as a strain of Atlantic Giant or perhaps
even visa-versa.
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
Family - Cucurbitaceae
Genus – Cucurbita
Species – Cucurbita maxima
Sub-species Variety – Atlantic Giant
Strain A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
Points to remember…
 Atlantic Giant and Show King giant squash are both of the species
Cucurbita maxima and differ only in color.
 Atlantic Giants will not cross with ‘true’ pumpkins.
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
MARRYING YOUR COUSIN
Plant geneticists broadly group plant reproduction into two categories:
inbreeding and outbreeding. Inbreeders are those who reproduce
independently of other like plants. Their male and female organs are
often contained within the same physical unit and require no third-party
means of pollen-to-egg transfer. Tomatoes are a good example of an
inbreeding type of plant. Outbreeders on the other hand, are
characterized by having two distinctly separate male and female flowers
and require an outside means to distribute the pollen between the two.
Commonly, bees, wind, or animals provides this means of transfer. All
cucurbits are outbreeders.
As you might expect, the progeny of inbreeders are mainly produced
through self-pollination while those of outbreeders are born through
cross-pollination. Both inbreeders and outbreeders technically may be
propagated through self and cross-pollination though inbreeders tolerate
self-pollination much better. This would seem natural given their
reproductive tendencies.
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
Points to remember…
 Plants reproduce either through inbreeding or outbreeding.
 Cucurbits are outbreeders.
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
HUH?
Within every plant is what’s known as genotypes. These genotypes
combine to make up the plant’s appearance, growth, and fruit
characteristics. The physical manifestation of genotypes are called
phenotypes. Size, shape, and color can all be classified as phenotypes.
Constructed of DNA and protein, one or more genes can represent each
phenotype and control its state or appearance. Going even deeper, alleles
serve as the alternate values for each gene. At least two are present at
any given time, one carried over from the mother, one from the father. In
any squash, we can have thousands of genes, all coming together to make
up the physical characteristics that we see. With this in mind, we can see
why an incredible number of variations in progeny are possible. Variance
in plants comes from the difference or heterozygosity of the alleles for
any given gene. Homozygosity is the sameness of these alleles and
manifests itself in the uniformity of the progeny. Self-pollination of
plants encourages homozygosity and enhances the predictability of the
offspring.
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
For each generation of self-pollination, the sameness of the allele pairs
becomes more frequent lending itself to more uniform offspring.
Cross-pollination introduces new gene/allele combinations and
therefore gives way to new variations in appearance and habit. These
offspring can display characteristics of either or both parents. The
probability of these characteristics or phenotypes depends on the
dominance and homozygosity of the genes responsible. These genes
can be dominant, codominant, incompletely dominant, or recessive
in nature. Crossing independent, homozygous varieties creates
heterozygosity or hybrids.
Note: When reviewing our hierarchy trees, many plant breeders would
consider these pumpkins siblings due to the close relationships many
of the pumpkins share.
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
Points to remember…
 Genotypes are made up of genes and combine to make up a plants
appearance and habit. Phenotypes are the physical manifestation of
the genotypes. Alleles represent the possible states for each gene.
 Homozygosity is the sameness or purity of the allele pairs.
 Heterozygosity is a difference in the allele pairs.
 Self-pollination encourages homozygosity. Cross-pollination
encourages heterozygosity.
 Genes can be dominant, codominant, incompletely dominant, or
recessive in nature.
 The crossing of two independent, homozygous lines creates
hybrids.
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
ANOTHER DEPRESSION
So why is everyone cross-pollinating rather than self-pollinating? If the
characteristics you desire are already there, why not just self-pollinate?
Why is everyone so eager to further complicate the gene pool?
Part of this unwillingness may have some basis in fact. As stated
previously, inbreeders naturally tolerate inbreeding or self-pollination.
Outbreeders often repel it causing what is known as inbred depression.
Poor vigor and even poorer subsequent seed production are the result.
Additionally, self-pollination often brings forth the homozygosity of
recessive genes. If these genes are negative in nature, the results can be
lethal or at least undesirable. Hence, many self- pollinated, outbreeding
strains fail to reproduce and carry on the strain.
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
Here’s the good news. Cucurbits do not seem to be influenced by inbred depression. Self-pollination appears to have no ill effects and
only serves to preserve the genetic qualities of the parent. In fact, tests
have been done to try and prove that self-pollination is detrimental to
the longevity of these plants and have failed. With this in mind, why
not use it to help preserve favorable traits in our favorite seeds?
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
Points to remember…
 Inbred depression can sometimes occur as a result of self-
pollination but has not been observed in cucurbits.
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
PRESS “F1” FOR HELP
‘F1 hybrid’. This is a term that is being tossed around quite a bit these
days in pumpkin circles but what exactly is it?
An F1 hybrid is the resulting cross of two inbred, homozygous lines.
The ‘F’ in ‘F1’ stands for filial, ‘1’ represents the first generation. A
monohybrid is a plant whose parents were heterozygous for only one
trait. A dihybrid is one whose parents were heterozygous for more than
one trait. All F1s should produce offspring that are uniform for the
trait(s) involved. Burpee’s Prizewinner Hybrid is a very good
example of an F1 Cucurbita maxima with the emphasis on shape and
color.
Many or you have probably heard the term ‘hybrid vigor’. What does
it mean? Do Atlantic Giants experience it?
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
Inbred lines, when crossed, sometimes experience something called
hybrid vigor. Abnormally vigorous plant growth and development may
characterize this. This hybrid vigor is only experienced in out-breeding
varieties but unfortunately, it has not been observed in the Cucurbita
maxima to any appreciable degree.
For those of you hoping to further test this, you would need to cross
two stable, inbred lines of Atlantic Giant and then grow the seeds of
the progeny. Simply cross pollinating between random Atlantic Giants
does not constitute conditions for hybrid vigor. Today’s Atlantic Giant
is a very aggressive grower by nature and capable of producing huge
fruit. Subsequently, detecting which plants were hybrid vigorous could
prove difficult to impossible.
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
It must be noted as well that hybrid vigor does not always manifest
itself in larger fruit. There are many, many hybrid vegetables on the
market today with the vast majority of them not being above-average
sized cultivars. Hybrid vigor could also mean a more aggressive
plant, better disease resistance, or greater seed production. To
summarize, hybrid vigor may be difficult to achieve or detect. Even if
successful, it does not guarantee a bigger pumpkin.
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
Points to remember…
 A hybrid is the result of crossing two homozygous lines.
 Hybrids can be either monohybrid or dihybrid.
 Hybrid vigor, characterized by abnormally aggressive plant or
fruit production, can sometimes occur as a result of hybridization but
has not been observed to any appreciable degree in cucurbits.
 The random crossing of two Atlantic Giant plants does not
constitute conditions for hybrid vigor.
 Hybrid vigor does not guarantee a larger fruit.
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
Alleles = T (dominant) and t (recessive)
t
t
T
Tt
Tt
t
tt
tt
50% = T
A.G.G.C.
50% = t
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
Example of an F1 cross
T
T
t
Tt
Tt
t
Tt
Tt
100% = T
A.G.G.C.
0% = t
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
Example of an F2 Cross
T
t
T
TT
Tt
t
Tt
tt
75% = T
A.G.G.C.
25% = t
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
MAGIC BEANS
Many growers seem to be under the impression the Atlantic Giant is
some magical variety that somehow transcends the laws of genetics. It
is not. In reality, it is a relatively simple, open-pollinated variety that
has been bred for size and little more. The laws of nature, genetics, and
physics most certainly apply and need to be observed. It is equally
important that we strive to find the simplest solutions possible and not
overcomplicate what are essentially simple tasks.
So how do we go about properly breeding the squash of our dreams?
The method best suited to the task is called genotypic recurrent
selection. This method involves the initial selection of seeds from a
favorable fruit, the growth and self-pollination of its seeds, and the
cross pollination of the next generation of progeny having the traits we
desire. The self-pollination between crossed generations is critical as it
will help to create sameness amongst the allele pairs. Without it, we are
basically nullifying the effects of the cross and further complicating the
equation.
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
¹Selection based on visual criteria alone with intercrossing is called
recurrent mass selection. The best improvement will be obtained
when parents are carefully chosen for each cycle. Experience has
shown that the improvement depends not only on the performance of
each plant on its own, but on how well it nicks with other parents in the
cross. Certain combinations simply work better than others, regardless
of the individual performance of the parents. The strategy used to
evaluate nicking ability is progeny testing. Progeny testing requires
saving seed of selected plants, and an extra season to evaluate their
progeny. If the progeny of a cross all perform well, the cross nicks well
and the seed from the original parents is used for the next cycle of
recurrent selection. To make the most of recurrent selection, selfpollinate the plants you plan to intercross the next generation. This will
double the response to selection by controlling the identity of the male
parent.
¹Farmer Cooperative Genome Project - http://www.fcgp.org/
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
“So what seeds should I be growing? What plant should I use as a
pollinator?”
I get questions like this, with no other information provided, emailed
to me all the time. No one can answer these questions for you without
knowing what your optimum goal is. Just plain big is not a sufficient
goal as most of today’s seeds have the firepower to knock out a few
1000+ pounders. Instead, you need to give some thought to which
phenotypes you deem desirable. Only then can you map out a strategy
in your search for or development of the ultimate seed.
Plant breeding is always done with a specific result in mind. Visualize
what you want your pumpkins to look like and start with seeds known
to produce similar fruit. Plan your crosses between plants exhibiting
the traits you seek. Focus more on breeding with proven, time tested
techniques and less on trying to find that magic cross. Random crosspollination produces random results. Importantly, don’t expect
immediate, favorable results. Plant breeding is largely trial and error
and strains can take many generations to perfect.
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
Points to remember…
 Genotypic recurrent selection, combined with careful self and
cross-pollination, is the best way to achieve our breeding goals.
 Follow time tested methods and procedures of plant breeding.
 Have an end result in mind before you start any cross-pollination
and start with lines already proven to produce progeny exhibiting the
characteristics you prefer.
 Don’t cross-pollinate just for the sake of doing so.
 Don’t be disappointed if you don’t immediately get the results you
desire.
 Keep it simple!
A.G.G.C.
© 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.
Most importantly…
Have
fun!!
Best of luck to all in 2004. May all your
pumpkins be big … and just slightly smaller
than mine. – Mike Nepereny 02/2004