DISCUSSION AND CONCLUSIONS

1
The Evolution of Placentas in the Fish Family
Poeciliidae the Southern Clade of Poeciliopsis
Ronald D. Bassar, Sonya K. Auer, Kevin E.
McBride, Grant Waltz and David N.
Reznick University of California - Riverside
RESULTS How did these Poeciliopsis species
differ in their life-history characteristics? A.
Univariate statistics show that there is
significant variation among species in the age at
maturity (males) (p lt 0.001), mean number of
offspring (plt 0.001), offspring size (dry mass
plt 0.001), interbrood interval (p lt 0.001),
reproductive allocation (p lt 0.001), and the
number of developing litters, or superfetation
(p 0.008).

• INTRODUCTION
• Within the fish family Poeciliidae there have
  been multiple, independent origins of the
  placenta (Reznick et al. 2002)
• Documenting the life historical context in which
  the placenta evolved is an important prerequisite
  to understanding its adaptive significance
• In a northern clade of the genus Poeciliopsis,
  the degree of matrotrophy was associated with a
  progressive decrease in age at maturity,
  offspring size, reproductive allocation and time
  interval between successive litters
• Placentation has evolved independently in a
  southern clade of Poeciliopsis
• We tested the generality of the life history
  trends found in the northern clade by examining
  the degree of placentation and associated life
  history traits in this southern clade

• DISCUSSION AND CONCLUSIONS
• Results for this preliminary study suggest that
• Matrotrophy, or post-fertilization maternal
  provisioning, is associated with an increase in
  the age at maturity (from male data), offspring
  size and interbrood interval but a decrease in
  reproductive allocation, brood size and degree of
  superfetation.
• Species with more highly developed placentas
  produced fewer, larger offspring later in life.
  This suggests that the evolution of the placenta
  in southern clade species may have facilitated
  the evolution of increased offspring size (Wourms
  and Lombardi 1992). Increased offspring size may
  be advantageous in environments with increased
  competition.
• The increases in age at maturity, offspring size
  and interbrood interval in matrotrophic species
  reported here stand in stark contrast to the
  pattern found in the northern clade of
  Poeciliopsis. However, a reduction in
  reproductive allocation was common to
  matrotrophic species in both clades. High current
  is thought to favor reduced reproductive
  allocation as a means of increasing swimming
  performance (Thibault and Schultz 1978 see also
  Marcelo Pires this conference in SAC 305, June
  26, 2006 830am). Thus, selection pressures
  caused by abiotic factors such as current may
  favor the evolution of the placenta and an
  associated decrease in reproductive allocation.
  Differences in biotic pressures such as predation
  may then explain the differences in traits such
  as offspring size and number found between the
  southern and northern clade of Poeciliopsis.
  Alternatively, how the placenta influences other
  life history traits may differ across its
  multiple origins. Finally, the evolution of the
  placenta may be completely independent of the
  evolution of other life history traits.

Table 1. Life-history comparisons among females
of species. 1The ratio between offspring size at
birth and egg size at fertilization 2The average
number of simultaneous broods found within
dissected females 3Average interbrood interval,
in days 4The percentage of the total body mass
attributable to the mass of embryos 5Average
offspring dry mass per brood, in mg 6Average
number of offspring per brood 7Average age at
which the gonopodium finished developing, in days
(males only) .
DEFINITIONS Lecithotrophy All or most nutrients
are delivered to young via a yolked egg, which is
retained, prior to fertilization. Initial egg
size is large. Matrotrophy All or most
nutrients are provided after fertilization.
Initial egg size is small. Superfetation The
ability to carry multiple broods in different
stages of development
B. Discriminant function analysis was performed
on offspring size, offspring number, interbrood
interval, reproductive allocation and
superfetation to evaluate the relationship
between these variables and matrotrophic index.
Function 2

• METHODS
• A preliminary, comparative study of
  closely-related species that exhibit up to a
  ten-fold difference in the amount of
  post-fertilization resource allocation to the
  embryos.
• We compared life history traits between two
  matrotrophic species (Poeciliopsis turneri and P.
  presidionis) and two populations of a
  lecithotrophic species (P. scarlli)
• Common-garden
• All variables taken from F1 females except age
  at maturity which is conservative across sexes
  within a species (see northern clade poster)

Figure 2. Canonical Discriminant Functions. Note
that function 1 separates species with different
degrees of maternal provisioning.
LITERATURE CITED Mateos, M. O.I. Sanjur, and
R.C. Vrijenhoek. 2002. Historical biogeography of
the livebearing fish genus Poeciliopsis
(Poeciliidae Cyprinodontiformes). Evolution 56
972-984. Reznick, D. N., M. Mateos, and M. S.
Springer. 2002. Independent origins and rapid
evolution of the placenta in the fish genus
Poeciliopsis. Science 218 1018-1020. Thibault,
R.E. and R.J. Schultz. 1978. Reproductive
adaptations among viviparous fishes
(Cyprinodontiformes Poeciliidae). Evolution 32
320-333. Wourms, J.P. and Lombardi, J. 1992.
Reflections on the evolution of piscine
viviparity. Amer. Zool. 32276-293.
Table 2. Canonical factor loadings for the first
two canonical functions
Matrotrophic
Lecithotrophic
Acknowledgements We thank Yuridia Reynoso for
her valuable help in the laboratory. This work
was supported by the DEB program of
Southern Clade
Table 3. Eigenvalue, percentage of variance and
canonical correlation for the first two canonical
discriminant functions.
Figure 1. Phylogeny of the genus Poeciliopsis
(Mateos et al. 2002), showing the three species
used in this study.