Transgeninc plants in Hawai'i: A new colonialism?

Hawai'i has the distinction of being the site of the first (and I think only) commercial trangenic fruit crop, a transgenic virus-resistant papaya. Without the transgenic papaya, there basically would be no Hawai'ian papaya industry. Even what little non-transgenic papaya production is left survives only by burying it in buffer zones of the transgenics.

Because of the level acceptance already in place for transgenic papaya, and probably because of the relative isolation of the island, Hawai'i is a popular plance for trials of transgenic crops. That's not real popular with some folks.

I'm a month late getting this posted, but Karl Haro von Mogel has a post on this up at Biofortified:

Hawai'i's curious relationship with with GE (Biofortified)

I can't remember if I've posted it before, but Karl's own blog, The Inoculated Mind is also worth a read.

Fruit Genetics Friday #7: Plant Sex Chromosomes Part II: Strawberries

A couple weeks back, in the very infrequent Fruit Genetics Friday series, I discussed sex chromosomes in fruit, specifically papaya. At the time, the only two fruit crops I knew of with sex chromosomes were papaya and kiwi. Monday I opened my e-mail to a paper, forwarded to me by a friend who is also one of the authors, describing a very interesting development in this area:

Genetic mapping of sex determination in a wild strawberry, Fragaria virginiana, reveals earliest form of sex chromosome (Heredity)

(Unfortunately, for non-subscribers, this will just be an abstract, but I'll try to hit the good stuff).

People were excited about the discovery of sex chromosomes in papaya, because it represented one of the most primitive sex chromosomes yet found in plants. Well, it turns out that F. virginiana, one of the ancestors of our cultivated strawberry, has sex chromosomes too (counter to what I said in that earlier post, incidentally), and these are even younger in their development! They're also really unusual in how they operate. In papaya and kiwi, as well as most other plants and lots of animals, the homogametic sex is the female (as in 'XX') and the heterogametic sex is male ('XY'). This seems to be the more typical arrangement, but strawberries, oddly enough, have the opposite arrangement.

So if you recall the arrangement in papaya, you had two critical loci. One, which we'll call "F" is a suppressor of femaleness, and the other is a promoter of maleness, which we'll call "M". So the two "wild type" chromosomes (which I'll call X and Y, since it puts it in terms people are familiar with and is essentially correct, even though the terminology isn't really used consistently in plants) consist of:

f---m (X chromosome)

F---M (Y chromosome)


So a female would be XX, or (fm)/(fm), so no maleness promoted and no femaleness suppressed. And male would be XY, with the femaleness suppressed, and maleness promoted. Hermaphrodites are generally a mutation of the Y chromosome (which we'll call the Y⁺ chromosome), so that the suppression of female development ceases to function, but male development still takes place:

(f)---M (Y⁺ chromosome)

So the sexes are basically female (XX), male (XY), and hermaphrodite (XY⁺). (because the YY isn't viable, you can't get a homozygous, true-breeding hermaphrodite).

Well, turns out strawberry has a different arrangement. The roles of the genes are kind of reversed. You have a dominant promoter of femaleness (which we'll call 'G' (for "gyn-", as used in the paper)), and a dominant gene for male sterility (which the paper calls 'A', as in "andro-"). And in this case we'll use Z and W (the system from birds) rather than the XY system.

So the arrangement is:

g---a (Z chromosome)

G---A (W chromosome)

So the sexes are female (ZW) and male (ZZ). This actually fits with some old research dating as far back as the 1920's suggesting that the female is the heterogametic sex. This was backed up by Ahmadi and Bringhurst, who suggested a single locus with three alleles, F, H, and M (in decreasing order of dominance). I kind of suspect many of these single locus, three allele systems, which have been proposed in other species as well, turn out to be variations on the two locus system, like in strawberry or papaya.

Here's a cool bit: The strawberry sex chromosome is pretty primitive, and so recombination in between these loci isn't fully suppressed, and you actually get crossover between them 5.7% of the time. As a result, you can get variations, namely hermaphrodites (resulting from a G/a recombinant) or a neuter (the result of a g/A recombinant). That suggests something which is only barely functioning as a sex chromosome.

Because the sex chromosome seems to have evolved very recently, what I'm curious about is how widespread in related species it is. I think it quite likely that the other major octoploid species, Fragaria chiloensis, and its offspring with F. virginiana, the cultivated F. x ananassa, share this scheme. But do the lower ploidy strawberries?

If diploid strawberries possessed sex chromosomes previous to the evolution of octoploids, then the octoploids should have eight sex chromosomes. Having this many would probably result in a mess (although it seems to work out alright for the short-beaked echidna), and the fact that sex inheritance seems to be pretty simple suggest that if the octoploid had eight sex chromosomes, then six of them have probably ceased to function as such.

Though there may be some exceptions (there are a bunch of east Asian diploid species I'm not really familiar with) the diploids I'm aware of, F. vesca, F. viridis, and F. nubicola seem to be almost (but not quite) uniformly hermaphrodite. Yet a number of higher ploidy species in addition to the octoploids, including the hexaploid F. moschata and the tetraploid F. orientalis are at least partially dioecious. It may be that polyploidy, by creating "backup" copies of the chromosomes possessing the sex controlling loci, allows divergence of one pair into more specialized sex chromosomes.

I'd be curious to know if closely-related genera such as Rubus and Potentilla share this system. Research suggests that in both of these species, the females are the heterogametic sex. If they possessed similar sex chromosomes, that would suggest that the strawberry's sex chromosomes have remained in their primitive state for a long time, or that something about the common ancestor was prone to the development of such an arrangement.

The other, perhaps less novel, but no less cool, aspect of the paper is that it introduces the first SSR-based map of the octoploid strawberry. Including two markers linked to the sex loci!

Anyway, all told, pretty cool, huh?

Fruit Genetics Friday #6: Sex and the Single Papaya

I came across this article while Googling for information on date palm sex inheritance the other night:

A primitive Y chromosome in papaya marks incipient sex chromosome evolution (Nature)

Unless you're a Nature subscriber, however, you can only read the abstract there, so try this for a more complete overview:

Scientists discover that papayas have sex chromosomes (University of Georgia)

It's an older article, but not one I'd seen. However it was not on date palm, so it was not immediately and directly relevant and I thought I'd wait until Friday to make it the launching point for another installment in this series.

The inheritance of sex in humans is probably one of the very first traits students learn the genetics behind (along with a vastly over-simplified and essentially incorrect description of the inheritance of eye color). It's a simple system, with a pretty distinct, binary (for the most part) phenotype, and it's something everyone cares about. Of course, sex isn't unique to humans. Plants and animals have distinct sexes as well--though sometimes they aren't quite so distinct. In fact, a majority of plants (and most fruit crops) are hermaphrodites (ie, they are simultaneously male and female), and there is evidence that suggests this is the ancestral state of plants as a whole--sex is a later invention.

Inheritance of sex in plants varies from species to species, and in fact may have evolved independently over a hundred times. It turns out, however, that an amazing lot of species have developed a similar system to people: females are homozygous (XX), and males are heterozygous (XY). Just like in humans, this will generally work out to a 1:1 ratio of male:female in the offspring. However, since sex is a little more flexible in many plants, some male plants will set the occasional fruit (interestingly, it virtually never happens the other way around, with male flowers on females). These self-pollinations show a 3:1 of males to females (or, occasionally 2:1, if the YY plants aren't viable). So we know that male is the heterozygous sex. Because of this, a locus denoted Su^F, for supressor of femaleness, was theorized. However, as the trait was further studied, it became clear, in part through cytological studies of chromosome deletion, that there were multiple loci involved, at least one or two more, involved in anther development. Hermaphrodites can result from mutation in one or the other of these genes.

In virtually all species, these genes are closely linked together. This makes sense, when you think about it: having "maleness" conditioned by multiple genes only works if you don't occasionally lose them to recombination. If a species is going to evolve separate sexes, and maintain them, these genes need to consistently work together to produce functional males. In some species these genes are an autosome (one of the standard, non-sex chromosomes) just like any other gene. However, despite the tight linkage, autosomes undergo crossing-over and recombination, so there will always been some risk of the linkage being broken up.

Enter the sex chromosome. Not all species that have sexes have sex chromosomes, getting by just fine keeping the required genes on an autosome. However, a sex chromosome has a major advantage, and that is to prevent recombination in the critical bit of genome. The human X and Y chromosomes may pair cytologically, but they're not two slightly different copies of the same thing the way the maternal and paternal copies of most chromosomes would be--there's vast differences in the content of the two (there would have to be given the size difference). I presume that's why the trait is denoted XX/XY not Xx/xx or Su^F/Su^f in people--we're really talking whole chromosomes assorting here, not just a trait.

(Note that although the XX/XY scheme is pretty common, it's by no means the only one out there. Birds have a ZW/ZZ scheme (just the same, but females are the heterozygous sex), and in some species there is no Y...just one or two copies of the X. The monotremes (such as the playtpus or the echidna) have one of the most bizarre schemes of all with multiple versions of both X and Y--for example in the short-beaked echidna it's XYXYXYXYX for males and XXXXXXXXXX (yes, there is one unpaired X in the male). I'm going to deal with XX/XY scheme exclusively for now, for fear of making things even less intelligible than they are now.)

Having two distinct sex chromosomes is useful in that by having two very different chromosomes, you suppress recombination, thus keeping things together appropriately. However, the trick is to have two different chromosomes that nonetheless pair up during meiosis, so that you always get one of each in each gamete. This can be accomplished because a portion of each sex chromosome remains similar (and thus capable of recombination). In people, a small area at one end of the X and Y chromosomes is identical and functions essentially as an autosome. Because this area does have a mate to seek out at meiosis, it pulls the non-pairing, non-recombinant bits along with it.

So how is recombination suppressed in the rest of it? The specifics aren't entirely clear, but basically the end result is that the Y chromosome ends up highly degenerate, accumulating deletions and mutations until, in some cases like humans, only a tiny handful of functional genes remain on it. That's probably why in many cases YY individuals aren't viable at all--they're missing functional copies of critical genes.

Plant Y chromosomes, it turns out, are on the whole not nearly as far gone as their mammalian counterparts. That's part of why people got excited about the papaya sex chromosomes. In papaya, normal recombination still occurs over 90% of the chromosome (compare that to less than 10% in humans), with only a small portion, which contains the male-determining genes, with evidence of heavy mutations and translocations. Because of this, it appears to be a sex chromosome in a very early stage of development, and provides a clear piece of evidence for the idea that sex chromosomes developed from autosomes. Recent work by geneticists shows that the non-recombinant region of the Y chromosome has a comparative lack of functional genes, and they estimate that the sex chromosomes evolved 1.3 to 2.8 million years ago, a mere twinkling of an eye in evolutionary terms.

Sex chromosomes appear to be more the exception than the rule in cultivated plants, however. One very good reason for this is probably the fact that most crop plants are hermaphrodites, and thus unlikely to be species with highly developed sex determination mechanisms. Some species (such as grapes) exist as males and females in the wild, but largely only as hermaphrodites in cultivated plants (the grape sex locus appears to be on an autosome). As far as I know, there are only two fruit crops that possess sex chromosomes: papaya (Carica papaya) and kiwi (Actinidia species). However, given how much more subtle the difference in previously described plant sex chromosomes are, there might well be some we have not yet discovered in fruits.

Papaya throws another wrinkle into the story by having hermaphrodites as well as male and female plants. Papaya cultivars, which are seed propagated, fall into two categories: those whose seeds give rise to male and female plants, and those that give rise to female and hermaphrodite plants (a majority of commercial cultivars are of that type, which makes sense, because who would want the non-fruitful male plants around). It appears that there may be two "flavors" of Y chromosome, one with a functional Su^F-type gene, and one without (but still possessing the necessary genes for anther development). This is supported perhaps by the fact it does not seem possible to generate a true breeding hermaphrodite papaya, perhaps because YY plants are not viable. In other words:

X_fX_f = female
X_fY_M = male
X_fY_H = hermaphrodite
Y_MY_M = not viable
Y_MY_H = not viable
Y_HY_H = not viable

Now that I've prattled on for a zillion words or so, I'd like to end on a brief note of caution...I'm not a papaya geneticist, so I may have gone horribly wrong in some of this, but this is the situation as I understand it.

The same goes for my understanding of kiwi, but I think it's interesting to note that although many kiwi species are polyploid, sex determination remains determined by a single gene pair, despite the sex chromosomes being duplicated (contrary to some early papers which suggested they were not, so that 2n=58=2x+XY but 2n=170=6x+XY...turns out that it's just really hard to count that many chromosomes...the hexaploids really had 174). So it looks as though there's a single pair of sex chromosomes that retains the ability to determine sex. (This also appears to be the case in octoploid strawberry, although the sex genes are on an autosome(s)).

Anyway, there it is. Probably the least exciting discussion of sex you've come across. (This will have to do for the time being instead of the persimmon sex post I mentioned in the comments a few days ago...I'm not sure of what became of it...I wrote it in Word because I was offline and on a plane at the time, and now I'm not sure where it is. Might be on the work laptop...(Sorry, Brandon.))