Viable Interspecific Eggplant Hybrids

The last year has been a mess. I'm fine. My family is fine. Most of my friends are fine. The increased anxiety and stress basically shut off any motivation or ability I had to write posts here. I was still active over on twitter or instagram, as those require less focused thought, but I just couldn't will myself to sit down at a computer and type up anything I felt was worthwhile to post.

I'm now fully vaccinated against covid19, but I know there are many people who still have not been able to access a vaccine. Some in my family. Many in the broader community. Covid cases in my community are dropping, but they're still higher than the peak we had in May of last year. I worry about recent CDC guidance and how people broadly seem to think it means the pandemic is over. It is not. Not here, and not elsewhere.

For now, most people locally seem to still be keeping up distancing and masking practices gained over the last year. As always, the next few weeks will be informative.

Even with the persistent writer's block, I routinely thought about writing something. This post is the first something to come of that. It isn't really the long and information or photo rich posts I like to write, but it is what it is

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My plant breeding projects have continued without interruption. My gardens have provided me with useful exercise and amusement.

Most of my plant breeding projects start with hybrids between divergent varieties within one species. The F1 generally stands out from the two parental lines, so Iit is fairly easy to have confidence that the cross took. In the F2 generation, there are almost always useful and unexpected traits which segregates out.

Last year I grew out a F2 population of scarlet eggplant. Every plant was different, but two stood out. One was extra productive and ripened fruit far earlier than any others. The other developed fruit that were white when immature, but ripened to the typical red later. This season I have F3 populations from those two plants.

I still haven't figured out how to like eating eggplant, especially the more bitter flavors of the scarlet eggplant, but I like the plants and will continue to try.

Recently I found some references describing successful hybrids between the scarlet eggplant (Solanum aethiopicum) and more common purple eggplant (S. melongena), with some significant effort in the lab. This got me thinking about what species one could make hybrids with among the eggplant. Any such hybrids would allow for much more diverse F2 populations, with their higher potential for selection towards interesting new traits.

This led to some discussion about primary (1'), secondary (2'), and tertiary (3') germplasm. 1' germplasm includes plants in the same or related species which can cross readily to your subject species. 2' germplasm includes plants which can cross to your subject species with significant reduction in fertility. 3' germplasm is then plants that can cross with your subject only with intensive laboratory operations such as embryo rescue or induced genome duplication.

In the case of eggplants, there has been much more exploration of 2' and 3' germplasm for the common eggplant. The scarlet eggplant is an important crop for many communities, but it has not attracted as much attention in communities with higher levels of biological research investment. As such, the 2' and 3' germplasm lists below for scarlet eggplant are very much incomplete.

 Asian Eggplant (Solanum melongena)

1. primary: S. incanum and S. insanum.
2. secondary: S. anguivi, S. dasyphyllum, S. lichtensteinii, S. linnaeanum, S. pyracanthos, S. tomentosum, and S. violaceum.
   
3. tertiary: S. elaeagnifolium, S. sisymbriifolium, S. torvum, and S. aethiopicum.
   

Scarlet Eggplant (Solanum aethiopicum)

1. primary: S. anguivi, S. macrocarpon, and S. dasyphyllum
   
2. secondary:
   
3. tertiary: S. melongena.
   

Professional plant breeders pursue traits from related species like these to improve disease resistance, drought resistance, or other traits important to growing large crops.

Independent plant breeders can afford to use traits from related species (among the 1' and 2' germplasm resources at least) to express their creativity towards developing new varieties. Even if you're not sure what to do with them (as I am), they're still lovely plants which might be fun to work with in the garden.

I hope you are and remain well as the pandemic continues.

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References

• S. melongena germplasm.
  
• https://journals.ashs.org/jashs/view/journals/jashs/141/1/article-p34.xml
• S. aethiopicum germplasm
• www.researchgate.net/publication/264393961_Conventional_and_phenomics_characterization_provides_insight_into_the_diversity_and_relationships_of_hypervariable_scarlet_Solanum_aethiopicum_L_and_gboma_S_macrocarpon_L_eggplant_complexes
• www.sciencedirect.com/science/article/abs/pii/S088915751500232X
• https://www.frontiersin.org/articles/10.3389/fpls.2017.01484/full 
• Fertility restoration in S. melongena x S. aethiopicum hybrids.
• https://link.springer.com/article/10.1023/B:EUPH.0000003883.39440.6d
  
• Primary, secondary, and tertiary gene pools.
  
• https://www.frontiersin.org/articles/10.3389/fpls.2014.00068/full

Botanizing in Hawaii: Solanum linnaeanum

Hawaii has a long history of biological invasions. Plants and animals from all over the world have arrived and thrived there under the tropical sun. This can make it somewhat difficult to identify a random plant, because it could literally come from almost anywhere on the planet.

On one of my hikes, I found several plants I immediately recognized a member of the family Solanacea. This is the same family that includes tomatoes, peppers, and eggplants. I was pretty sure it was even a member of the genus Solanum, one of the several species sometimes referred to as "spiny eggplants".

 I collected a few fruit, intending to secure some seeds for planting back home in Minnesota. A couple days later, while sorting through my collection at a motel, I finally identified the plant as Solanum linnaeanum. Among its various common names are: "Poison Apple", "Devil's Apple", and "Apple of Sodom". They're native to parts of southern Africa, but have naturalized in Hawaii and various other places around the world.


The plant and its fruit are chock-full of toxins. Enough so that very few animals are willing to eat it. At about this point I decided not to take seeds home for garden trials. I do have a few seeds in my collection that come from highly toxic, or otherwise dangerous, plants. I wasn't entirely certain how much difficulty I would have with trying to leave Hawaii with collected seeds. If some official asked me what they were and why I had them, they might not appreciate my responses. So, to limit that risk, I dumped the S. linnaeanum seeds in the garbage.

(Several days later when I left the state, I learned I would have had no trouble at all. Dried seeds in vials didn't concern the USDA officials at the airport at all. Bringing seeds into Hawaii gets their attention, not taking seeds away from Hawaii. Next time I'll be a bit more bold.)

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Interestingly, it appears this species can cross with domesticated eggplant (S. melongena). Doganlar et al. studied an F2 population derived from a cross of the two species in order to map the genomic positions of genes for traits important in domestication. Unfortunately, their paper doesn't have any photos of what the F2 plants looked like. Even with the risk of dragging the toxic traits from S. linnaeanum into the progeny, this would be a fun cross to recreate and explore. The wild species probably has numerous disease/insect resistance traits that would be useful in a garden eggplant, so there is probably value to the experiment beyond simple personal amusement. There's a few online vendors offering seed for this species, so I won't have to make another trip to Hawai'i to start on this project.


References:

• http://www.starrenvironmental.com/images/search/?q=Solanum+linnaeanum
• https://www.ctahr.hawaii.edu/invweed/WeedsHI/W_Solanum_linnaeanum.pdf
• https://en.wikipedia.org/wiki/Solanum_linnaeanum 
• http://www.genetics.org/content/161/4/1713

Doom of the Fruit Fly

The red-eyed fruit-fly familiar to biology students everywhere (Drosophila melanogastor) likes to lay eggs in recently spoiled fruit, where its larvae can consume the fruit sugars and the yeasts that grow on them. If you have an infestation of D. melanogastor in your kitchen, the first step is to get rid of whatever they're breeding in. The second step is to get rid of all the adults that are flying around. You could spray some poison, get skilled with a fly-swatter, or buy some commercial traps... but it is a pretty simple task to make your own trap without needing any poison. Make a paper cone with a small hole in the tip, then put it point-down into small jar with a bit of over-ripe fruit in the bottom as bait

The inverted cone prevents the fruit flies from finding their way out once they've crawled inside. If you don't have an inverted cone, you'll just be breeding more fruit flies to infest your kitchen. The bait can be whatever rotten fruit was attracting the flies.

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One species of fruit-flies that doesn't like recently spoiled fruit is the Spotted-Wing Fruit Fly (Drosophila suzukii). This species lays its eggs in ripe soft fruit (raspberries, strawberries, blueberries, etc.). They are able to do this because the females have a saw-edged ovipositor that lets them cut into soft fruit to lay their eggs. They don't have to wait for fruit to begin rotting like the typical fruit flies and their infestation will result in the rapid destruction of fruit that would otherwise go to market. This has become a big problem for people trying to grow berries without pesticides. There are traps for these flies, but they're only really useful as a surveillance tool. The traps alone will not protect your fruit.

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Image from blogs.cornell.edu post.

Another group of fruit-flies that don't like recently spoiled fruit are the dark-eyed fruit flies. D. repleta, D. hydei, and D. robusta are slightly larger and prefer their food source to be far more degraded. What this often means is the grimy collection of goo in floor drains or between/underneath cracked floor tiles in restaurant kitchens. It also can mean chicken poop in a barn, a mostly rotted compost pile, or the litter of a reptile tank (that I really probably should have cleaned already).

Once I noticed the existence of the flies, I cleaned up the source of the problem and my gecko is happy with her spiffy new home. The fruit-fly trap I described for D. melanogastor would work equally well for these flies if I could figure out a bait that they would be attracted to. I could scoop some of the old rotted reptile media into the jar, but I discovered something a bit more interesting (and effective).

S. integrifolium "Pumpkin Tree" fruit.

I had the branches and fruit from a Solanum integrifolium "Pumpkin Tree" plant I grew this year hanging up in our kitchen to dry. I first noticed the dark-eyed fruit-flies because they were hanging out on the branches when I checked on how the dry the fruit was. As an experiment, I put a couple of the fruit into the previously described fruit-fly trap and put the remainder of the plant outside.

The next morning, all of the flies from the kitchen were having a party -inside- the trap. I scattered all the flies outside and reset the trap beside the (now clean) reptile tank. The next morning, all the fruit-flies in that area were also inside the trap. The dark-eyed fruit-flies (of whichever species I have here; D. repleta, D. hydei, or D. robusta) really, really, love the fruit of the "Pumpkin Tree" plant.

I've dehydrated and powdered the remaining fruit to save them for later use as a bait. If the dark-eyed fruit-flies ever reappear, I'll mix some of the powder with some water as bait and continue the experiment. I could also do some experiments outside next summer, as the dark-eyed fruit flies are likely to prosper in my compost pile again.

I've saved seeds from the "Pumpkin Tree" plant and will definitely be growing it again for its decorative and fly-trapping features.

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References:

• Drosophila melanogastor
• en.wikipedia.org/wiki/Drosophila_melanogaster
• Drosophila suzuki (Spotted Wing Drosophila)
• www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74158.html
• Dark-eyed Drosophila
• insectsinthecity.blogspot.com/2010/02/dark-eyed-fruit-flies.html
• blogs.cornell.edu/nysipm/2015/06/30/fruit-flies-of-a-different-eye-color/
• Solanum integrifolium "Pumpkin Tree"
• the-biologist-is-in.blogspot.com/2015/09/novel-vegetable-scarlet-eggplant.html

Novel Vegetable: Scarlet Eggplant

S. integrifolium "Pumpkin Tree".

Solanum integrifolium (or S. aethiopictum, the nomenclature is a bit confused) goes by a few common names depending on what variety you're working with. A type that is grown primarily for decorative use in the USA is referred to as the "Pumpkin Tree" (or "Pumpkin-On-a-Stick") because of the ripe fruit have the color and shape (if not size) of pumpkins. The orange-red color of the fruit is characteristic for the species. Many other red/orange eggplants you will find are varieties of the same species.

Most photos of "Pumpkin Tree" plants that you find online have lovely deep purple-black stems, which provide nice contrast to its bright orange-red fruit when dried and used in a floral display. My plant has the green stems typical of your everyday eggplant, marking it as somewhat distinct from most of the photos I come across. My seeds were saved from a dried fruit that I found in a bouquet a friend had brought over from her florist employer. I didn't note the color of the stems, but I'm assuming they were light in color.

Sautéed "Pumpkin Tree".

The fruit are widely reported as edible, though they're more bitter than the usual cohort of American vegetables. There are references to the vegetable tasting better before it has turned orange, so I decided to prepare a pair of simple dishes to compare them. The ripe and unripe fruit from the top photo were diced and then sautéed separately with olive oil and a dash of salt and ground long-pepper. The ripe fruit was squishier and made more of a mess on my cutting board than the unripe fruit. As the diced eggplant was cooking, I was surprised to find many of the seeds were popping like popcorn, leaping out of the skillet in the process. (They didn't gain much in size like popcorn, however.)

After each dish was done, I sampled a small bite. Between trying one and the other, I rinsed out my mouth with some milk. The mature fruit might have been slightly more bitter than the immature fruit, but both were far more restrained than either had been when tasted raw. The bigger issue for me was the grainy texture of all the popped seeds. I may try preparing a batch of them into a stir-fry or some other dish that would prevent the popped seeds from being so prominent in the final meal.

The fruit are a bit too small for easy preparation. A larger-fruited variety, like several sold at www.rareseeds.com, would be a better place to start if you're interested in this plant as a vegetable. [I have no association with www.rareseeds.com, but I do think they're a cool company.]

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I have seeds for another variety of scarlet eggplant that I found at a Hmong farmer's market stand near the University of Minnesota. The fruit were small, round berries that were connected in chains (like how cherry tomatoes often are). I chatted up the salesperson about the fruit, as I had never seen them before and was curious about them. He said the elders' name for the plant was [Hmong]"Iab Lws" (I had to have him spell it out, as I don't speak the language.) and that it is culturally considered the ideal pairing with squirrel. The name translates roughly to "Bitter Berry", which is a perfectly descriptive name because the berry is rather bitter. I wasn't planning to buy any, but in appreciation for my interest in his crop and culture, he insisted I take a large bundle as a gift. I don't typically enjoy bitter vegetables, but I've started to appreciate their place in cuisine.

It isn't yet clear to me if "Iab Lws" is another example of S. integrifolium (or S. aethiopictum) or if it represents the ancestral species S. anguivi. I planted several seeds early in the spring, but the plants that came up got lost along the way. I do have a single plant of "Iab Lws" that was started roughly mid-way through the summer. Though small, it has been growing well and is now flowering. Hopefully it will develop fruit before the first fall frost. Even if it doesn't fruit, I should be able to at least get some nice photos of the plant.

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Since "Iab Lws" and "Pumpkin Tree" are very closely related, they should cross very easily. Though many gardeners take great pains to ensure their different varieties don't cross, so they can preserve the varieties as historical artifacts, I tend to be a bit more flexible. It doesn't bother me if my varieties cross. In fact, my varieties crossing helps to ensure that what I grow every year will remain interesting. My "Iab Lws" has started flowering and I've moved it to be directly adjacent to my "Pumpkin Tree" plant in hopes of that a cross might happen. I could perform the cross manually, but right now it is more convenient for me to simply setup situations where insects will do the work of crossing for me.

"Iab Lws" and "Pumpkin Tree" are distinct enough (as mature plants) and there is no reason to expect that all alleles responsible for visible traits in one variety would be dominant over the alleles from the other variety. It should be relatively simple to identify any resulting hybrids early enough to isolate them and ensure self-pollination (of at least the later fruit). I'm really interested in what recombinations of the traits from the two varieties would turn up in the F2 population and in subsequent generations. Large fruit developing in elongated chains would be really cool (as well as useful).

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S. integrifolium "Korean Red".

I recently came across a fruit for yet another S. integrifolium variety, "Korean Red". This variety has been bred primarily as a vegetable and the fruit are much less bitter as a result. The fruit is definitely denser than those of "Pumpkin Tree", suggesting to me that it would be much more useful as a kitchen ingredient. I'm holding off from cutting into this one because I want to let it mature its seeds for me to grow next year.

When the fruit had become very shrunken, I cut it open and extracted the seeds for final drying. The fruit residue smelled very sweet, without any of the bitterness I was expecting. I didn't actually taste the residue, so I'll have to wait for next year to tell if the scent was telling the truth about the fruit.

References:

• Pumpkin Tree:
• www.walterreeves.com/gardening-q-and-a/pumpkin-tree/
• www.pumpkinnook.com/howto/pumpkinonastick.htm
• www.etsy.com/listing/209307454/pumpkin-tree-solanum-integrifolium-fun
• gardenvarietynews.wordpress.com/2013/11/15/pumpkin-on-a-stick-ornamental-eggplant/
• www.learn2grow.com/plants/solanum-integrifolium/
• freshbynorthwest.com/?p=3711
• Other varieties of S. integrifolium:
• www.rareseeds.com/aubergine-du-burkina-faso-eggplant/ 
• www.rareseeds.com/cookstown-orange-eggplant/ 
• www.rareseeds.com/goyo-kumba-eggplant/
• www.rareseeds.com/korean-red-eggplant/
• www.rareseeds.com/melanzane-rosso-di-rotonda/
• www.rareseeds.com/red-china-eggplant/
• www.rareseeds.com/red-ruffled-eggplant/
• www.rareseeds.com/turkish-orange-eggplant/
• Something similar to "Iab Lws":
• www.plantnames.unimelb.edu.au/new/Sorting/CATALOGUE/Pt1-African-eggplants.html#Solanum-anguivi
• S. anguivi:
• www.prota4u.org/protav8.asp?p=Solanum+anguivi
• www.prota4u.org/plantphotos/Solanum%20anguivi%204.jpg
• Relationships of Scarlet Eggplants:
• journal.frontiersin.org/article/10.3389/fpls.2014.00318/full