The Fertile Families - and Beyond

The concept of a Fertile Family, members of which produce more members of the same family when intercrossed, has been used by hybridizers for decades to describe the more intricate workings of wide crosses.  Most hybridizers work with just one fertile family (like the TBs).  Aril/arilbred hybridizers, however, have three families available (diploid arils, tetraploid arils, and amphidiploidlike arilbreds).  While each affords ample opportunity for traditional line-breeding, collectively they are a tempting realm to explore.  

In its most precise form, a separate symbol is used for each aril species.  The more general version, however, recognizes only aril and bearded chromosome sets and makes no distinctions among the many different types in each group.  This simplifies to only 12 types involved in producing arils and arilbreds. 

In the following chart, "A" stands for one set of aril chromosomes and "B" stands for one set of bearded chromosomes.  

AA

That may still seem like a lot, but when these crosses are charted they sort themselves out quite neatly into manageable categories. 

In the chart below, the parental types are shown as both row and column labels.  If you pick a column corresponding to one type and a row corresponding to the other, the cell where they intersect gives the results of that cross – the possibilities, not the expected ratios.  Fertile families are shown in light green and the relatively infertile intermediate types in yellow.  Other color codes are explained in the categorical descriptions below the chart.  Clickable links lead to the corresponding Photo Gallery.

The symmetry of the chart may not be immediately obvious because the cells aren't square, but if we also ignore the direction of the cross we need to deal with only 78 different types of crosses that fall into five categories.  The simplest cases, crosses within type, appear along the diagonal.

Crosses Within the Fertile Families

Five of these are crosses within one of the familiar fertile families:  the diploid arils, tetraploid arils, amphidiploidlike arilbreds, diploid beardeds, and tetraploid beardeds.  All offspring from such a cross have the same type of chromosome complement as their parents, which makes them by far the easiest to work with. Like their parental types, these are shown in light green. They yield a high percentage of takes and produce full pods.  Their seeds exhibit relatively good germination and their seedlings have highly predictable results. Entire programs can be conducted by working within one of these cells and ignoring all other possible combinations.

Crosses Within a Relatively Infertile Type

The other seven along the diagonal are crosses between two members of the same relatively infertile group.  Because neither parent is fully fertile, the chances of getting any offspring at all are slim.  In fact, it was this contrast that led many to label the intermediate types as “sterile” when the concept of Fertile Families became widely used.  Intermediate types like this, however, are capable of producing two different types of gametes – so such a cross can produce three types of offspring: the parental type or either of the ancestral types. These crosses are shown in brown.

Crosses Between the Fertile Families

Ten more of the cells show the results of crosses between two of the five fertile families.  They also have predictable results because all of the offspring from a cross between two such families have the same type of chromosome complement.  Of these, one produces offspring that are members of a fertile family other than that of either parent (shown in green), while the other nine produce relatively infertile intermediate types (shown in yellow).  Some of these have also been quite popular because they reliably produce valued garden subjects like 1/4-breds and 3/4-breds. A great deal can be accomplished without venturing beyond this category into the world of more complex crosses.

Crosses Between a Fertile Family and a Relatively Infertile Type

Thirty-five are crosses between a member of one fertile family and a relatively infertile type.  These are somewhat more complicated than working within or between fertile families because the relatively infertile partner produces two types of gametes and the seedlings thus fall into two types. The real challenge comes in evaluating the seedlings for possible registration because sibs may have different chromosome complements and thus belong to different registration classes. Although this group is considerably more difficult to work with, there is enough fertility to make it worthwhile for the adventuresome.  These cells are shown in dark green. 

Crosses Between Two Different Relatively Infertile Types

The final 21 are the widest type of cross, where the odds are greatly against getting any offspring at all.  Each parent produces two types of gametes.  Because the two parents do not necessarily produce any gametes of the same type, however, some very interesting recombinations can occur.  For example, crossing a triploid aril with a triploid bearded could produce four types of arilbreds: an AB-type diploid, ABB- and AAB-type triploids, or an AABB-type amphidiploid.  These crosses are shown in gray.