Trait comparisons with other anadromous fishes
Despite strong differences in philopatry and population genetic
structure, there are similarities between the Pacific lamprey ecotypes
and those of steelhead trout (anadromous Oncorhynchus mykiss ) and
Chinook salmon (O. tshwaytscha ), as were described by Parkeret al. (2019). In this study, we observed even greater
similarities with steelhead of these ecotypes in the Willamette River
than were apparent in the Klamath River Pacific lamprey. Notably, like
steelhead (Hess et al. 2016), the Pacific lamprey ocean- and
river-maturing ecotypes exhibit seasonal separation where premature fish
return to freshwater long before spawning in contrast to mature fish
that arrive shortly before spawning. Also similar to steelhead
(Micheletti et al. 2018), the Pacific lamprey ocean-maturing form
is only distributed in coastal regions and the river-maturing ecotype is
distributed further inland. However, it is unknown whether inland
migrating Pacific lamprey exhibit both early and late arrival to
spawning grounds as observed for inland migrating steelhead (Micheletti
et al. 2018) and Chinook salmon (Narum et al. 2018). Finally, although
we found no evidence that the homologous genes were conserved with
salmonids, Pacific lamprey ecotypes were associated with a single locus
of major effect as shown in steelhead (Hess et al. 2016;
Micheletti et al. 2018) and Chinook salmon (Prince et al. 2017; Narum et
al. 2018).
The adult body size trait that was associated with chromosome 02 genes
in Pacific lamprey may share similarities with the age-at-maturity trait
described in Pacific salmon (McKinney et al. 2019), steelhead
(Copeland et al. 2017), and Atlantic salmon (Salmo salar ,
Barson et al. 2015). In salmonids, the number of consecutive
years spent in the ocean before returning to freshwater as a mature
adult is highly correlated to body size (e.g., Chinook salmon, Lewiset al. 2015). It has also been shown that larger, older Chinook
and Sockeye salmon (O. nerka ) tend to arrive earlier at
Bonneville Dam compared to the smaller 1-ocean-age adults (Anderson and
Beer 2009). Similarly, larger Pacific lamprey arrive earlier at
Bonneville Dam than the smaller bodied forms (Keefer et al. 2009,
2013), which may be related to life history decisions in seasonal
environments. The primary growth of Pacific lamprey occurs during the
ocean phase of the lamprey’s parasitic life cycle and so bigger lamprey
may also be older in ocean age. There is not yet an accurate way to
measure the total age or ocean age of lampreys since they lack bony
structures, but this hypothesis could be tested once an aging method is
developed (e.g., statolith microstructure). Collectively, the
convergence of traits in salmonids and lamprey suggest strong tradeoffs
between allocation of resources in capital breeding fishes, whereby long
distance migration constrains maturation schedules and in at least some
cases (e.g., lamprey) body size.