Hamilton and D’Artagnan: A tale of two plant breeding projects

Years ago, I bought an interesting-looking melon with intriguing longitudal grooves from a street vendor in Paris and tasted for the first time the exotic, intoxicating flavor of the Charentais. Around the same time back home in his Massachusetts garden, my twin brother was tending a patch of his favorite crop: butternut squash. These actions were the inspiration for two plant breeding projects focused on improving the flavor of the foods that we grow and eat.

Why Undertake a Plant Breeding Project?

For most people who really enjoy a particular fruit or vegetable, it is enough to purchase one from the grocery store or local farmer and savor eating it. Others, in order to get improved quality and to participate more fully in the experience, grow their own in their garden and may even save the seeds of their favorite heirloom for next year. I see plant breeding as taking this process a step further, or maybe even several steps. To develop a new variety through years of patient, attentive work is to understand a crop in an even deeper way and to become part of the process. To create a variety that has the attributes that you have picked out, to shape it through artificial selection, to eventually give it a name, is to put a bit of yourself into the variety and to make it your own.

A more practical reason to start a plant breeding project is because there may not be a cultivar available to grow that meets the criteria you are looking for. Perhaps you are growing in a unique climate, or looking for a variety with great flavor, and are unable to find any suitable seeds. Plant breeding is a way to create for yourself the thing you are looking for.

Plant breeding can also allow us to take control of our seed security. The seed industry has become extremely consolidated into a few multinational corporations who supply the vast majority of the seeds that we grow. This means that we rely heavily on these few entities to supply us with cultivars that meet our diverse needs. As you might expect in such a scenario, plant breeding projects from these companies are focused primarily on the largest seed markets such as production farms in California and Mexico. The new cultivars they are developing may not always work well for organic growers in the Northeast, for example. To make matters worse, the older legacy varieties that they have acquired may not have the sales volume to support continued production and are often discontinued. By developing and becoming stewards of our own new varieties we can offer alternatives to this corporate system and return some level of control to gardeners and smaller acreage growers.

Importantly, plant breeding can be a lot of fun as well. Luther Burbank, the famed Massachusetts-born plant breeder of the late 19th and early 20th centuries, once said, “What a joy life is when you have made a close working partnership with Nature, helping her to produce for the benefit of mankind new forms, colors, and perfumes in flowers which were never known before; fruits in form, size, and flavor never before seen on this globe.” I must hasten to add a bit of warning. Plant breeding can be somewhat addictive. It is an exercise in delayed gratification – a typical plant breeding project takes five years or more to complete – and the anticipation each season of the harvest can become something of an obsession. Only at harvest do you see the progress you have made since the last season, and waiting for that day can preoccupy your thoughts throughout the rest of the year. It’s a fixation that tends to escalate with the time, effort, and many small failures and successes that happen along the way. So please, proceed with caution.

A Range of Complexity

Plant breeding projects range from fairly simple to complex. The potential size and scope of a project is only limited by the genetic variation available to us and by our own imagination. Developing an open-pollinated cultivar with good flavor and local adaptation can be fairly straightforward and is something that can be done by passionate plant breeding hobbyists who need not quit their day job to do so. Other plant breeding projects, especially those involving the development of high yielding and disease resistant hybrids, or that involve wide crosses to wild species, are a bit more involved and are typically undertaken by those few of us who are devoted to this line of work full time.

‘Hamilton’ open-pollinated butternut and ‘D’Artagnan’ hybrid Charentais melon are examples of two new cultivars bred to have good flavor. They represent some of the range of complexity that is possible when developing a new plant cultivar, and hopefully will serve to illustrate some of the steps required in this process.

Getting Started

Achieving an understanding of the floral biology and pollination techniques of a particular crop is vital before you begin, because all new plant breeding projects typically begin the same way by crossing two existing cultivars together. The original parental plants should be chosen to contain characteristics that you would like to combine together, such as different flavors, shapes, maturity, colors, disease resistances, etc. After this initial cross, multiple generations of planting, pollination, selection and seed saving are performed in the attempt to identify and stabilize the new variety you have envisioned.

The methodologies used to make pollinations vary widely between different crops, and there are several resources that do a good job explaining the finer points of each species. Butternut squash (Cucurbita moschata) and Charentais melon (Cucumis melo) are both members of the Cucurbitacea Family and are outcrossing species – that is, their floral biology tends to promote pollination between plants as opposed to individual plants pollinating themselves. This means that, unless steps are taken to prevent it, a flowering butternut plant may be pollinated by any other butternut plant in the vicinity. This can be a problem if you are trying to select and stabilize a particular set of characteristics because your plants may be pollinated by undesirable individuals. In order to control pollination, two things must occur.

First, bee pollinators must be prevented from visiting your flowers. This can be achieved by securing the flowers closed the day before they open or by excluding pollinators by doing your work in a greenhouse. Second, pollen from the desired plant (which may or may not be the same plant that is receiving the pollen) must be delivered by hand to the awaiting flower.

The butternut squash, to me, is a quintessentially Yankee crop. It is a humble, respectable vegetable. It is solid and dense – a hardy, steadfast fruit that waits patiently on the vine until the gardener has time to bring in the harvest. Butternut squash are put away at summer’s end and reemerge from the cellar when the nights are cold and the days are short as a winsome reminder of the summer sun. It rarely takes center stage on the kitchen table but adds a savory-sweet accompaniment to hearty winter meals.

Butternut squash has been a staple in my brother Derek’s Massachusetts garden for many years. By the 2014 season he had settled on a commonly-grown powdery mildew resistant F1 hybrid as his preferred variety and decided to try his hand at developing his own open-pollinated version. The fairly simple idea of this project was to “dehybridize” his favorite butternut hybrid – that is, to develop a stable, open-pollinated cultivar by inbreeding while selecting for, and possibly improving on, the flavor and disease resistance of the starting hybrid. This methodology is a great way for a beginning plant breeder to get started, and a more detailed description of the process can be found in Carol Deppe’s book Breed Your Own Vegetable Varieties (2000).

F1 hybrid cultivars are the result of crossing two butternut lines together, so the first step of making an initial cross pollination had already been performed. Consequently, all that was required in the first season was to grow out a crop of the hybrid cultivar without any other butternuts in the area. Simple enough. Plants of a hybrid variety are nearly genetically identical to each other, so it didn’t matter which plant(s) contributed the pollen. At the end of the season, we extracted seeds from the fruit and put them away until the spring.

The seeds harvested from the F1 hybrid cultivar make up the F2 generation. This is an exciting stage because all of the genetic differences that exist between the starting parental plants have been recombined and shuffled like a deck of cards. Consequently, each F2 plant is a genetically distinct individual. The F2 plants were grown out in Derek’s 2015 garden – which can only fit about 25 butternut plants. That year, every plant in the garden was pollinated to itself. Squash plants are monoecious, meaning there are female flowers and male flowers on the same plant. To pollinate a squash plant to itself requires securing the flowers closed prior to opening using a twist tie or other method, and returning the next day to hand deliver the pollen between flowers. This must be done on every plant at flowering stage before you know which among them will produce desirable fruit. After pollination was completed, the plants were monitored for resistance to powdery mildew and attractive fruit shape. The selected fruit were put down cellar until Christmas time, when we made another round of selection based on how well they stored. For the last stage of selection, we relied solely on a taste test.

This was the fun part. Each fruit was labeled individually and the blossom end of the fruit, which contained the seeds, was set aside while the rest of the squash was cooked in the oven. We then sat down with a group of friends and family and tasted each butternut fruit, like wine tasters in Napa Valley, carefully noting on paper the texture qualities, flesh color, and assigning a flavor rating. At the end of the tasting, the notes were compiled and the winning fruit was selected. Of course, the seeds from that fruit were the ones chosen to be planted out the following year.

In the summer of 2016 the F3 generation, which came from the best F2 fruit, was grown in the garden. This time, because the individual plants were more closely related than the previous year, there was less variability between them. We decided to allow these plants to open-pollinate each other to prevent too much inbreeding and to avoid having to make time-consuming hand pollinations. Again, selections were made in the garden for disease resistance, fruit shape, and yield. Again, selected fruit were stored in the fall and selected for flavor. The seeds from the best two squash were planted out in the summer of 2017. This process was repeated a final time in 2018. By this season we were happy with the results: a uniform, medium-sized butternut with dark orange flesh, great flavor, good storage, and resistance to powdery mildew. The variety was named ‘Hamilton’ after the town in Massachusetts where it was developed – a nod to the famous ‘Waltham’ butternut of Waltham, Mass.

Development of ‘D’Artagnan’ Charentais Melon

The Charentais melon is the dramatic, epicurean cousin of the butternut squash. A Charentais melon shouts its presence with a fragrant, musky aroma that quickly fills a room and cannot be ignored. The fruit is soft and sweet and must be eaten shortly after picking. It is a summer flirtation, a fleeting indulgence, and is unavailable outside of harvest season. It is a dessert unto itself and, even when draped with rich prosciutto or drizzled with port wine as is customary in France, the melon is the star of the show.

When I tried my first Charentais it was like I had been struck by lightning. I had never tasted anything like it. The initial perception is a cotton candy sweetness that is quickly overtaken by a bloom of floral volatiles that expands into your sinuses and feels like it crawls up inside your skull. A good Charentais should almost make your eyes water. As the taste fades, you are left with a lingering buttery flavor like burnt-caramel. After that first experience, eating a melon from my local grocery store was like holding a candle to the sun and I’ve been on a mission to develop my own variety for the US market ever since.

One of the major challenges of working with this type of melon is that, since they are not commonly grown in the US, it is difficult to get Charentais seeds to start with. By happenstance, years ago I came across some seeds of a Charentais heirloom at the old Lockhart Seeds store in Stockton, California and grew them out. The melons had the flavor that I remembered, but the fruit were extremely small and the very late maturity and lack of disease resistance made them difficult to grow. This gave me the idea to develop a parental line that could compensate for these problems with the hope that a hybrid between them might combine the best of both worlds.

I realized that a large, early-maturing Charentais with high yield and disease resistance was needed to complement the great tasting heirloom I had found. In most melon growing regions of the US, it is desirable to have resistance to Powdery Mildew and 3 races of Fusarium Wilt, so I made crosses between disease-resistant melons and the few Charentais types that I could find and began the long process of inbreeding by hand-pollinating hundreds of melon plants. This project was similar to the butternut breeding project but on a larger scale, and, because the end goal was to produce a hybrid, the breeding lines were self-pollinated every generation to make them highly inbred and uniform. I have many projects running at the same time, so I do my work in a greenhouse to exclude pollinators and make hand pollination easier. In order to select for multiple disease resistances, I also had to employ some additional techniques.

During the first few generations of this process I hired a lab that offers a genetic fingerprinting technology service, much like 23andMe for plants. I sent them small pieces of leaf tissue and for just a few dollars per plant they extracted the DNA and scanned the genetic sequence for genes that encode for disease resistance. This technique, called Marker Assisted Selection, can be very useful when trying to combine multiple resistances into a single cultivar, especially when screening for resistance visually is difficult. Marker Assisted Selection is only an option for common crops where researchers have successfully identified and described genes of interest. Fortunately, melon is one such crop. Only my plants that contained genetic resistance were potted up, hand-pollinated, and rated at harvest time for fruit shape, size, appearance, and flavor. The seeds from plants with good fruit characteristics were collected and planted out again the following season.

During later generations I worked with two talented college student interns to verify the disease resistance by challenging my plants with fungal pathogens. After obtaining fungal spores from the USDA and growing them in an incubator, we inoculated the melon plants and noted which became sick and which did not. We continued growing out resistant families and discarded those that were susceptible.

To develop inbred lines that are highly uniform and stable typically takes about eight generations of inbreeding. After years of work and intensive selection I developed nine disease-resistant inbreds, most of them derived by crossing Charentais and American cantaloupes, that I was happy with. I was ready to make hybrids with the flavorful heirloom that had been waiting for its partner all this time. The heirloom variety was crossed to all nine new breeding lines to make nine new F1 cultivars with the hope that, in at least one of these combinations, the desirable characteristics from each parent would be contained in the hybrid between them.

These new experimental hybrids were grown out with farmers in multiple locations over two years to see how they performed under different growing conditions. They were grown in New England, Florida, Arizona, California, Oregon, and Washington State. One of the most enjoyable steps for me is visiting the different growing regions to evaluate the fruit at harvest. Walking, knife in hand, through a field with many different varieties and stopping to look, feel, smell, and taste each one is a lot of fun. Knowing the history behind each line, the years of growing them out, pollinating, extracting seeds, replanting, etc. adds to the excitement as you see how they combine with each other. There are always a lot of surprises at this stage, as it can be difficult to predict which crosses will make the best hybrid and often the parental characteristics combine in unexpected ways. The best approach is to make many F1 hybrid crosses and evaluate how they perform under many growing environments over multiple years.

During the trialing process I was looking primarily for the one that had the best flavor and yield. My hope for this variety is that it will be a sort of ambassador to help popularize Charentais in the United States, so it also had to have the classic size, shape, and color. After two years of testing in multiple growing regions I was able to narrow it down to my favorite hybrid. I named the variety ‘D’Artagnan’ after Alexandre Dumas’s character in The Three Musketeers. Dumas, in addition to being one of my favorite authors, had his own obsession with the Charentais melon. In 1864 he donated a copy of all of his published works to the town of Cavaillon, France in exchange for an annuity of 12 melons each year.

In the early spring of 2018, more than half a decade after I’d eaten my first Charentais, I sent seeds of ‘D’Artagnan’ to one of the largest growers in the South of France. In July I traveled there to see the trial, timing the trip to coincide with “La Fete du Melon” harvest festival. My brother Derek joined me on the trip. On a sun-drenched morning we visited the melon farmer in the production area south of Nimes.

There, among flowering fields of lavender and the Mediterranean marshes, field after field of Charentais melons sprawled across the French countryside. After carefully navigating meandering farm roads in our rental car, we pulled up to the field where the farmer was trialing new varieties from different seed production companies from around the world. There, beside dozens of other new hybrids, was a plot of ‘D’Artagnan’ – the maturing fruit poking through the wide leaves like treasures. As we walked into the field the farmer counted the number of fruit per meter and nodded his head approvingly. Still, I could see that he strongly doubted that an American would know anything about the Charentais melon, which is a subject of great French national pride. As we cut the first one and ate it I tasted the familiar burst of sweetness, followed by a wave of perfume, and couldn’t help but smile. It had taken a long time to get here, but I finally had my own Charentais melon variety. I watched the farmer as he took a bite. I was happy to see his eyes widen in surprise.

Derek Cavatorta, PhD, DVM is a large animal veterinarian and avid vegetable gardener who lives in Hamilton, Massachusetts. Jason Cavatorta, MS, PhD is President and Plant Breeder of EarthWork Seeds Inc., a vegetable breeding research and development company based in Oviedo, Florida. Seeds of ‘Hamilton’ organic OP butternut and ‘D’Artagnan’ F1 Charentais melon will be available from seed retailers for the 2020 growing season.