A
Mussel Hybrid Zone:
Genetics,
Ecology and Evolution
Implications
for Aquaculture
Mussel research at Memorial University of Newfoundland
Investigators:
Dr. David Innes, Department
of Biology, Memorial University
Dr. Raymond Thompson, Ocean
Sciences Centre, Memorial University
Cyr Couturier, School of
Fisheries, Marine Institute, Memorial University
Current Students:
Shin Kim (PhD)
Previous students:
Dr. Quangxu
Liu (PhD)
Dr. J. Ben Lowen (PhD)
Dr. Marcelo Miranda (PhD)
Dr. Jorge Toro (PhD)
Jennifer Bates (MSc)
Background: The Mytilus edulis - Mytilus trossulus hybrid zone
Blue mussels are important
for aquaculture in Atlantic Canada where two species (Mytilus
edulis and M. trossulus)
coexist and hybridize along the
coasts of Newfoundland, Nova Scotia, New Brunswick and Québec. Both species are
morphologically very similar and
can only be reliably distinguished using genetic markers. A major question
facing the
mussel aquaculture industry is
the relative performance of the two species and the affect of hybridization on
production.
Despite the absence of
sufficient experimental data, there is a perception that M. trossulus is less desirable for
aquaculture. If true, this
information will affect most mussel aquaculture sites in Atlantic Canada where
a mixture of the
two species occurs. Our studies
on wild populations in Trinity Bay, Newfoundland suggested that M. trossulus has a
higher mortality rate and
shorter life span than M. edulis.
Our research group has been
working on the mussel hybrid zone in Newfoundland for a number of years. We
are continuing our research in order
to better understand the genetic and environmental factors maintaining
this hybrid zone.
Newfoundland:
Rationale:
Aquaculture requires a
database of scientific knowledge for improving production.
Academic researchers offer much
scientific experience and expertise, and there is a broad
overlap between academic
research and aquaculture interests. In addition, information on the
production characteristics of
aquaculture species requires a number of years of research.
Therefore, improvements to
aquaculture production require a long-term investment in research.
The mussel hybrid zone in
Atlantic Canada offers an excellent opportunity for basic scientific
research on how mussel ecology,
genetics and hybridization can provide useful information
to improve mussel aquaculture.
Methods:
Diagnostic
genetic markers
Mussel species cannot be easily
distinguished based on morphology. Species and hybrids can only
be reliably distinguished using
diagnostic genetic markers. Genetic markers available consist of
enzyme protein variation as well
as DNA variation (nuclear and mtDNA). Genotype can be
determined
from a small piece of tissue and
each individual classified as M. edulis, M. trossulus or hybrid.
hybrid M. edulis M. trossulus
(ITS fragments separated in an agarose
gel. Photo: J. Toro)
Shell colour and morphometrics
Once mussel species have been
identified using genetic markers, the species can be examined for
any difference such as shell
morphology and colour. There is a tendency for M. trossulus
to have a
darker inner shell compared to M.
edulis at some sites in Newfoundland.
M. trossulus
M. edulis
A canonical variates analysis based on several shell characteristics
shows that
M. edulis
and M. trossulus are morphologically distinct. However, the
presence of
hybrids blurs this distinction. (Figure from J. Toro,
1999)
Change in genetic composition with shell length and life history stage
Genetic markers have been
used to examine the change in relative frequency
of the two mussel species and
hybrids among mussels differing in shell length.
M. trossulus
individuals dominate
the spat and smaller size classes while M. edulis
is most frequent in the larger
size-class mussels. Hybrids occur with low frequency
for all size classes. This
pattern suggests that M. trossulus has a
greater mortality
and slower growth rate compared
to M. edulis. M. edulis
can therefore grow to
a larger size. (Figure from Toro,
Innes and Thompson, in review)
Larvae
Samples of larvae just prior
to settlement showed a high frequency of M. trossulus
and a lower frequency of M. edulis and hybrids with mixed genotypes based on two
genetic markers. First
generation hybrids (F1) occurred with the lowest frequency. These
observations suggest that
factors controlling the production of hybrids occur earlier in
the life history than the
pre-settlement larval stage.
Variation in the reproductive cycle
Measurements of the
reproductive cycle (Gamete Volume Fraction) show that M. trossulus
has a
greater investment in gametes
than either M. edulis or hybrids. The results
also show that M. edulis
spawns (drop in GVF between June
and July) more completely than M. trossulus
and hybrids.
This temporal difference in
spawning may reduce the probability of hybrids being produced.
Other experiements
show that gamete incompatibility and mortality during the early embryo
stage also play a role in
reducing the probably of hybridization (Miranda, in prep.).
(Figure from
Toro, Thompson and Innes, 2002)
Growth at Aquaculture sites
Groups of mussels of known
species composition have been produced in the
laboratory by pooling families
of M. edulis, M. trossulus
and hybrids. Two types
of hybrids were produced with
either M. edulis (Hybrid Ed) or M. trossulus
(Hybrid Tross)
acting as the female parent. These mussel have been
growing
at two different aquaculture
sites. The results to date show that M. edulis had
the greatest shell growth with M. trossulus
and Hybrid Tross showing the
smallest shell growth. (data from M.
Miranda and B. Lowen)
M. trossulus
M. edulis
Condition Index
M. trossulus showed a significantly greater
condition index than M. edulis. This is
consistent with M. trossulus having a greater investment in reproduction
than
M. edulis. (Data from B. Lowen)
Mortality
Hybrids showed the greatest
mortality. (Data
from B. Lowen)
Summary:
The careful control of species composition using genetic markers has allowed experiments
to document life history
differences between the coexisting mussel species M. edulis,
M. trossulus and
their hybrids. These life
history differences likely play an important role in maintaining the coexisitence
between the two species. Furthermore,
both pre- and postzygotic isolating mechanisms
prevent
extensive hybridization between
the species. Further research is in progress to determine genetic
and ecological factors that
maintain this mussel hybrid zone. These studies can also provide information
on performance characteristics
that can assist in increasing mussel production at aquaculture sites
by favouring one species or the
other.
Mussels also show a great
deal of variation in shell colour and shape, some of which is under genetic control.
Shell colour in mussels has a
simple genetic basis:
Contact for further information: David Innes (dinnes@mun.ca)
Papers:
Liu G.X., Stapleton E., Innes D.J.,
Thompson, R.J. (2011) Aggregational
behavior of the blue mussels Mytilus edulis
and Mytilus trossulus:
a potential pre-zygotic reproductive isolation mechanism. Marine Ecology 32: 480-487. http://onlinelibrary.wiley.com/doi/10.1111/j.1439-0485.2011.00446.x/abstract
Liu
G.X., Innes D.J., Thompson R.J. (2011) Quantitative analysis of sperm plane
circular movement in the blue mussels Mytilus edulis, M. trossulus
and their hybrids. Journal
of Experimental Zoology PART A- Ecological Genetics and Physiology, 315A
(5): 280-290. http://onlinelibrary.wiley.com/doi/10.1002/jez.674/abstract
Miranda M.B.B., Innes D.J., Thompson R.J.
(2010) Incomplete
reproductive isolation in the blue mussel (Mytilus edulis
and M. trossulus) hybrid zone in the Northwest
Atlantic: Role of gamete interactions and larval viability. Biological Bulletin,
218: 266-281. http://www.biolbull.org/content/218/3/266.full?sid=b06a9774-8172-42fd-a4bd-b3c2e795d4d8
http://www.mun.ca/biology/dinnes/Incomplete_Reproductive_Isolation_in_the_Blue_Mussel.htm
Toro, J. E., R. J. Thompson and D. J. Innes (2006) Fertilization success and early survival in pure and hybrid larvae of Mytilus edulis (Linnaeus, 1758) and M. trossulus (Gould, 1850) from laboratory crosses. Aquaculture Research 37: 1703-1708
Toro, J. E., D. J. Innes
and R. J. Thompson (2004) Genetic variation among life-history stages of
mussels in a Mytilus edulis –M. trossulus
hybrid zone. Marine Biology 145, 713 – 725 (download PDF file)
Toro, J. E., R. J.
Thompson and D. J. Innes (2002) Reproductive isolation between coexisting Mussel
species (Mytilus edulis, M. trossulus) in
Newfoundland. Marine Biology 141, 897–909.
(download
PDF file)
Comesana, A. S., Toro, J. E., Innes,
D. J. & Thompson, R. J. (1999) A
molecular approach to the ecology of a mussel (Mytilus edulis - M. trossulus) hybrid
zone on the east coast of Newfoundland, Canada. Marine Biology 133, 213-221. (download PDF file)
Innes, D. J. & Bates, J. A. (1999) Morphological variation of Mytilus
edulis and Mytilus trossulus in eastern Newfoundland. Marine Biology 133, 691-699. (download PDF file)
Bates, J. A. & Innes, D. J. (1995) Genetic variation among
populations of Mytilus spp. in eastern Newfoundland. Marine
Biology 124, 417-424.