Flexibility in Timing of Ovarian Differentiation in Lampreys May Correspond with Feeding Type Flexibility

Non-parasitic lampreys, which do not feed after metamorphosis, have smaller body sizes at maturation and therefore lower fecundity than parasitic lampreys.  Previous studies have suggested that this fecundity difference is established in the larval stage through ovarian differentiation at a younger age (and smaller size) in non-parasitic females—and that feeding type is thus established at ovarian differentiation.  However, evidence of facultative parasitism and a failure to detect genetic differences between some parasitic–non-parasitic pairs has led to suggestions that feeding type may not be fixed in all species.  Our tendency to extrapolate among species pairs fails to recognize that different pairs represent speciation at different stages.  We thus compared the timing of ovarian differentiation in non-parasitic Northern Brook Lamprey Ichthyomyzon fossor and parasitic Chestnut Lamprey I. castaneus in southeastern Manitoba, Canada, and found no significant differences in timing between these non-parasitic and parasitic species.  We conclude that Northern Brook Lamprey, particularly in recently deglaciated areas, diverged very recently from parasitic Silver Lamprey I. unicuspis and has not yet become fixed for earlier ovarian differentiation.  This is consistent with the lack of genetic differentiation observed between these two species, and may suggest a concomitant flexibility in adult feeding type.