Breeding European Black Elderberries (Sambucus nigra)

 There are many wonderful named selections of European black elderberries (Sambucus nigra) selected for either productivity of berries or for ornamental use.  One area that has been underdeveloped despite the great potential is the selection of dual purpose elderberries which combine both ornamental traits such as cut-leaf/lace-leaf and or purple or variegated foliage with larger berry size and increased self fertility to ensure heavy fruit set.

  In order to pursue these goals it is useful to understand the genetics controlling the various traits.  Following is what I have been able to determine:

• Cut-leaf/lace-leaf type - When crossing two cut-leaf elderberry clones I found that 100% of the resulting offspring expressed this trait.  The first true leaves to develop after germination didn't fully express this morphological variation, but as seedlings became well developed the foliage uniformly became heavily dissected like the parent plants.  Alternatively, when I crossed a cut-leaf type to a standard broad-leaf type the resulting seedlings expressed broadleaf type only.  In terms of probability this indicates that the cut-leaf trait is most likely controlled by a single recessive gene.
• Purple foliage - When crossing standard green leaved to purple leaved types I've found that the offspring appear green for most of the growing season, but take on an increasing level of purple tinge to the leaves as the summer winds down and cool weather sets in.  This indicates that it's not simply a dominant or recessive trait, but rather a partially co-dominant trait.  It appears that seedlings possessing only one copy of the gene can produce enough pigment that they can at least be identified as carriers of the gene at certain points during the year, but for most of the growing season they do express very little color other than green.  

Above: Potted seedlings with a purple laciniate leaved mother and a green typical leaved father.  The result is normal leaf shape in all the seedlings (none are laciniate), but all have purple pigment in the spring which begins fading as the season progresses.

• Self-fertility - As yet I have not been able to determine if any of my seedlings possess any degree of self-fertility, but in assessing my parent plants of various ornamental and production type clones I have noticed a clear trend.  Clones selected primarily for ornamental traits have tended to have low or no fruit set unless they are growing very close to a different clone which provides cross pollination.  This indicates to me that within this species self-incompatibility may be very common in populations not intentionally selected for high productivity.  The small number of non-ornamental clones I grow that were selected purely for fruit productivity have proven to set fruit reasonably well even if they are moved away from other clones to limit likelihood of cross pollination.  While I do not have any data yet to indicate whether self fertility is dominant, recessive, co-dominant or even controlled by multiple genes, what is clear is that strict selection for high production seems to automatically favor the selection of self fertile clones. 
• Variegation - Variegated leaves in elderberries stem from having two types of tissue within the same plant; tissue with ability to produce chlorophyll and tissue without the ability to produce chlorophyll.  Since most variegated clones have the chlorophyll free zones along their margins this should result in seed embryos developing primarily from tissue with ability to produce chlorophyll and therefore the resulting seedlings would not be any more likely to be variegated than seedlings of non-variegated parent trees.  On the off chance that tissue without chlorophyll becomes an embryo the offspring would likely be albino and quickly die unless the embryo got lucky enough to contain both tissue types to produce variegation.  Interestingly, Hosta breeders have found that Hosta cultivars with stable variegation typically produce non-variegated seedlings and Hosta "breeder type" cultivars with unstable/randomly placed variegation often produce variegated seedlings at a much higher rate.  This is because the random placement of the variegation results in a higher percentage of embryos developing from intermediate areas containing both tissue types.  In black elderberries there is a clone called S. nigra 'Pulverulenta' which has mottled green and white leaves with an exceptionally high rate of transition back and forth between chlorophyll containing and albino portions of tissue.  Therefore, this clone would likely be a great choice for use as the seed parent if attempting to breed new variegated selections of black elderberry.  It could be particularly interesting to get a variegated clone that also produces purple pigment which I suspect would appear pink in areas of the leaves which lack chlorophyll.  This could potentially be very attractive, but would be challenging due to the incomplete dominance of the gene which causes purple pigmentation in black elderberry.  Even if the pigment were only to show faintly during cool months, it could still be very garden worthy.
• Golden/Aurea foliage - I have no elderberries of this type to work with so I have no findings at this time.
• Berry color - While "black" elderberries are almost always black or purple, there is a cultivar called 'Goldbeere' which produces pale translucent green berries with no trace of black or purple pigment.  Additionally, I have observed that the dark purple, cut-leaf selection 'Eva' (aka "Black Lace") produces fruit with pigment matching the dark leaves, but without the saturated clear black or purple of common types.  For this reason I hypothesize that 'Eva' may also be without normal pigments in its fruit, but only appear colored as a bleed over from the purple pigment that colors its leaves and stems.  This supposition is re-enforced by the fact that the fruit from 'Eva' possesses this color for the entirety of its development and does not start off green like other black elderberries.  While attractive, this trait presents a challenge to visually determining ripeness since color is no indication.  At this time I have no experience to know the heritability of the trait for lacking normal pigment production in the fruit, but it is something I would generally want to select against for two reasons.  First, it would be nice to still have a clear color change at ripeness in ornamental purple leaved types, and second the pigments which cause the black or purple coloration in black elderberry fruit are considered to be major parts of what makes them healthy for humans to consume!

This is all the information I have on this topic for now, but I will update this blog post if/when I learn more.