[2] CMS has also been identified in one animal species so far, Physa acuta, a fresh water snail.
[3] There is strong evidence for gynodioecy and CMS to be a transitionary step between hermaphrodites and separated sexes.
Natural selection on cytoplasmic genes could also lead to low pollen production or male sterility.
CMS differs from the latter case (nuclear male sterility) because most cytoplasmic genetic elements are only transmitted maternally.
[5] While CMS is controlled by an extranuclear genome, nuclear genes may have the capability to restore fertility.
Incorporation of these systems for male sterility evades the need for emasculation in cross-pollinated species, thus encouraging cross breeding producing only hybrid seeds under natural conditions.
One path, emasculation is done to prevent a plant from producing pollen so that it can serve only as a female parent.
For crops such as onions or carrots where the commodity harvested from the F1 generation is vegetative growth, male sterility is not a problem.
In the early 1970s, plants containing CMS-T genetics were susceptible to southern corn leaf blight and suffered from widespread loss of yield.
[11] Unfortunately, these lines are prone to environmentally induced fertility restoration and must be carefully monitored in the field.
[12] The systematic sequencing of new plant species in recent years has also uncovered the existence of several novel nuclear restoration of fertility (RF) genes and their encoded proteins.
A unified nomenclature for the RF defines protein families across all plant species and facilitates comparative functional genomics.
This nomenclature accommodates functional RF genes and pseudogenes, and offers the flexibility needed to incorporate additional RFs as they become available in future.